diff --git a/exercises/26_Bluetooth_discover/scripts/exercise26_ble_rssi_plotly.pl b/exercises/26_Bluetooth_discover/scripts/exercise26_ble_rssi_plotly.pl
new file mode 100644
index 0000000..e9e5e79
--- /dev/null
+++ b/exercises/26_Bluetooth_discover/scripts/exercise26_ble_rssi_plotly.pl
@@ -0,0 +1,626 @@
+#!/usr/bin/env perl
+# 20260526 ChatGPT generated
+# $Id$
+# $HeadURL$
+#
+# sudo ln -s /home/jlpoole/work/perl/exercise26_ble_rssi_plotly.pl /var/www/localhost/cgi-bin/exercise26_ble_rssi_plotly.pl
+# chmod +x /home/jlpoole/work/perl/exercise26_ble_rssi_plotly.pl
+#
+# Example browser URLs:
+#   /cgi-bin/exercise26_ble_rssi_plotly.pl
+#   /cgi-bin/exercise26_ble_rssi_plotly.pl?receiver=BOB
+#   /cgi-bin/exercise26_ble_rssi_plotly.pl?receiver=BOB&value=avg_rssi
+#   /cgi-bin/exercise26_ble_rssi_plotly.pl?pairs=BOB:CY,CY:BOB,ED:FLO,FLO:ED
+#   /cgi-bin/exercise26_ble_rssi_plotly.pl?pairs=BOB:CY,CY:BOB,ED:FLO,FLO:ED&value=avg_rssi&gps_valid=1&clock_valid=1
+#   /cgi-bin/exercise26_ble_rssi_plotly.pl?receiver=FLO&gps_max_ms=5000&bucket_s=5
+#
+# Console test:
+#   REQUEST_METHOD=GET QUERY_STRING='receiver=BOB&value=avg_rssi' ./exercise26_ble_rssi_plotly.pl > /tmp/rssi.html
+#   ./exercise26_ble_rssi_plotly.pl 'receiver=BOB&value=avg_rssi' > /tmp/rssi.html
+#   ./exercise26_ble_rssi_plotly.pl receiver=BOB value=avg_rssi > /tmp/rssi.html
+#
+# Purpose:
+#   Emit a Plotly HTML graph from an Exercise 26 BLE Discovery SQLite database.
+#   X axis is receiver observation time. Y axis is RSSI or avg_rssi.
+#
+# Database selection:
+#   1. DB_BLE_RSSI_PLOTLY environment variable
+#   2. DB_BLE environment variable
+#   3. hard-coded default below
+#
+# Required Perl modules:
+#   DBI
+#   DBD::SQLite
+#   JSON::PP
+#
+
+use strict;
+use warnings;
+
+use DBI;
+use JSON::PP qw(encode_json);
+
+my $DEFAULT_DB_FILE = '/home/jlpoole/work/tbeam/ble/ble_fieldtest_20260526_1859.sqlite';
+my $DB_FILE = $ENV{DB_BLE_RSSI_PLOTLY} || $ENV{DB_BLE} || $DEFAULT_DB_FILE;
+
+# Prefer a locally staged Plotly file so the graph works without depending on a CDN.
+# Override in Apache or shell with, for example:
+#   export PLOTLY_JS_URL=/lib/plotly-2.35.2.min.js
+my $PLOTLY_JS_URL = $ENV{PLOTLY_JS_URL} || '/lib/plotly.min.js';
+
+my @UNITS = qw(AMY BOB CY DAN ED FLO GUY);
+my %VALID_UNIT = map { $_ => 1 } @UNITS;
+
+my %q = parse_query_string();
+
+my $receiver    = uc($q{receiver} // '');
+my $heard       = uc($q{heard}    // '');
+my $pairs_text  = uc($q{pairs}    // '');
+my $value       = lc($q{value}    // 'rssi');
+my $gps_max     = $q{gps_max_ms};
+my $gps_valid   = $q{gps_valid};
+my $clock_valid = $q{clock_valid};
+my $bucket_s    = $q{bucket_s};
+my $limit       = $q{limit};
+
+if ($receiver ne '' && !$VALID_UNIT{$receiver}) {
+    emit_error(400, "Bad receiver value");
+}
+
+if ($heard ne '' && !$VALID_UNIT{$heard}) {
+    emit_error(400, "Bad heard value");
+}
+
+if ($value !~ /^(?:rssi|avg_rssi)$/) {
+    emit_error(400, "value must be rssi or avg_rssi");
+}
+
+if (defined $gps_max && $gps_max ne '' && $gps_max !~ /^\d+$/) {
+    emit_error(400, "gps_max_ms must be an integer");
+}
+
+for my $flag_name (qw(gps_valid clock_valid)) {
+    my $v = $flag_name eq 'gps_valid' ? $gps_valid : $clock_valid;
+    next unless defined $v && $v ne '';
+    if ($v !~ /^[01]$/) {
+        emit_error(400, "$flag_name must be 0 or 1");
+    }
+}
+
+if (defined $bucket_s && $bucket_s ne '' && $bucket_s !~ /^\d+$/) {
+    emit_error(400, "bucket_s must be an integer number of seconds");
+}
+$bucket_s = 0 unless defined $bucket_s && $bucket_s ne '';
+$bucket_s = int($bucket_s);
+if ($bucket_s < 0 || $bucket_s > 3600) {
+    emit_error(400, "bucket_s must be between 0 and 3600");
+}
+
+if (defined $limit && $limit ne '' && $limit !~ /^\d+$/) {
+    emit_error(400, "limit must be an integer");
+}
+$limit = 0 unless defined $limit && $limit ne '';
+$limit = int($limit);
+if ($limit < 0 || $limit > 1_000_000) {
+    emit_error(400, "limit must be between 0 and 1000000");
+}
+
+my @pairs = parse_pairs($pairs_text);
+
+if (!-f $DB_FILE) {
+    emit_error(500, "Database not found: $DB_FILE");
+}
+
+my $dbh = DBI->connect(
+    "dbi:SQLite:dbname=$DB_FILE",
+    "",
+    "",
+    {
+        RaiseError     => 1,
+        AutoCommit     => 1,
+        sqlite_unicode => 1,
+    }
+);
+
+my $traces = fetch_rssi_traces(
+    $dbh,
+    receiver    => $receiver,
+    heard       => $heard,
+    pairs       => \@pairs,
+    value       => $value,
+    gps_max     => $gps_max,
+    gps_valid   => $gps_valid,
+    clock_valid => $clock_valid,
+    bucket_s    => $bucket_s,
+    limit       => $limit,
+);
+
+my $summary = fetch_summary(
+    $dbh,
+    receiver    => $receiver,
+    heard       => $heard,
+    pairs       => \@pairs,
+    value       => $value,
+    gps_max     => $gps_max,
+    gps_valid   => $gps_valid,
+    clock_valid => $clock_valid,
+);
+
+$dbh->disconnect;
+
+emit_html($traces, $summary);
+exit 0;
+
+sub fetch_rssi_traces {
+    my ($dbh, %arg) = @_;
+
+    my @where = (
+        'rx_epoch_ms IS NOT NULL',
+        'receiver IS NOT NULL',
+        'heard IS NOT NULL',
+        "$arg{value} IS NOT NULL",
+    );
+    my @bind;
+
+    if ($arg{receiver} ne '') {
+        push @where, 'receiver = ?';
+        push @bind, $arg{receiver};
+    }
+
+    if ($arg{heard} ne '') {
+        push @where, 'heard = ?';
+        push @bind, $arg{heard};
+    }
+
+    if (@{ $arg{pairs} }) {
+        my @pair_terms;
+        for my $p (@{ $arg{pairs} }) {
+            push @pair_terms, '(receiver = ? AND heard = ?)';
+            push @bind, $p->{receiver}, $p->{heard};
+        }
+        push @where, '(' . join(' OR ', @pair_terms) . ')';
+    }
+
+    if (defined $arg{gps_max} && $arg{gps_max} ne '') {
+        push @where, 'gps_fix_age_ms <= ?';
+        push @bind, int($arg{gps_max});
+    }
+
+    if (defined $arg{gps_valid} && $arg{gps_valid} ne '') {
+        push @where, 'gps_valid = ?';
+        push @bind, int($arg{gps_valid});
+    }
+
+    if (defined $arg{clock_valid} && $arg{clock_valid} ne '') {
+        push @where, 'clock_valid = ?';
+        push @bind, int($arg{clock_valid});
+    }
+
+    my $select_sql;
+    my $order_by;
+
+    if ($arg{bucket_s}) {
+        my $bucket_ms = $arg{bucket_s} * 1000;
+        $select_sql = qq{
+            SELECT
+                receiver,
+                heard,
+                CAST((rx_epoch_ms / $bucket_ms) AS INTEGER) * $bucket_ms AS plot_epoch_ms,
+                AVG($arg{value}) AS plot_rssi,
+                COUNT(*) AS row_count,
+                MIN(rssi) AS min_rssi,
+                MAX(rssi) AS max_rssi,
+                MIN(vbat_mv) AS min_vbat_mv,
+                MAX(gps_fix_age_ms) AS max_gps_fix_age_ms,
+                MIN(clock_valid) AS min_clock_valid,
+                MIN(gps_valid) AS min_gps_valid
+            FROM ble_observation
+            WHERE } . join(' AND ', @where) . qq{
+            GROUP BY receiver, heard, plot_epoch_ms
+        };
+        $order_by = ' ORDER BY receiver, heard, plot_epoch_ms';
+    }
+    else {
+        # Important: do not mix aggregate functions such as COUNT/MIN/MAX with
+        # non-aggregate row columns here. SQLite will otherwise collapse the
+        # result to one arbitrary row, which looks like a single blue dot.
+        $select_sql = qq{
+            SELECT
+                receiver,
+                heard,
+                rx_epoch_ms AS plot_epoch_ms,
+                $arg{value} AS plot_rssi,
+                1 AS row_count,
+                rssi AS min_rssi,
+                rssi AS max_rssi,
+                vbat_mv AS min_vbat_mv,
+                gps_fix_age_ms AS max_gps_fix_age_ms,
+                clock_valid AS min_clock_valid,
+                gps_valid AS min_gps_valid
+            FROM ble_observation
+            WHERE } . join(' AND ', @where) . qq{
+        };
+        $order_by = ' ORDER BY receiver, heard, rx_epoch_ms, obs_id';
+    }
+
+    my $sql = $select_sql . $order_by;
+
+    if ($arg{limit}) {
+        $sql .= ' LIMIT ?';
+        push @bind, $arg{limit};
+    }
+
+    my $sth = $dbh->prepare($sql);
+    $sth->execute(@bind);
+
+    my %series;
+    while (my $r = $sth->fetchrow_hashref) {
+        my $key = $r->{receiver} . ' hears ' . $r->{heard};
+
+        push @{ $series{$key}{x_ms} }, 0 + $r->{plot_epoch_ms};
+        push @{ $series{$key}{y} },    numeric($r->{plot_rssi});
+        push @{ $series{$key}{customdata} }, [
+            $r->{receiver},
+            $r->{heard},
+            $r->{row_count},
+            $r->{min_rssi},
+            $r->{max_rssi},
+            $r->{min_vbat_mv},
+            $r->{max_gps_fix_age_ms},
+            $r->{min_clock_valid},
+            $r->{min_gps_valid},
+        ];
+        $series{$key}{receiver} = $r->{receiver};
+        $series{$key}{heard}    = $r->{heard};
+    }
+
+    my @traces;
+    for my $key (sort keys %series) {
+        push @traces, {
+            name       => $key,
+            mode       => 'lines+markers',
+            type       => 'scatter',
+            x_ms       => $series{$key}{x_ms},
+            y          => $series{$key}{y},
+            customdata => $series{$key}{customdata},
+            hovertemplate => join('',
+                '<b>%{customdata[0]} hears %{customdata[1]}</b><br>',
+                'time=%{x}<br>',
+                'RSSI=%{y:.1f} dBm<br>',
+                'rows=%{customdata[2]}<br>',
+                'rssi range=%{customdata[3]} to %{customdata[4]}<br>',
+                'min vbat=%{customdata[5]} mV<br>',
+                'max gps age=%{customdata[6]} ms<br>',
+                'clock_valid=%{customdata[7]} gps_valid=%{customdata[8]}',
+                '<extra></extra>',
+            ),
+        };
+    }
+
+    return \@traces;
+}
+
+sub fetch_summary {
+    my ($dbh, %arg) = @_;
+
+    my @where = (
+        'rx_epoch_ms IS NOT NULL',
+        'receiver IS NOT NULL',
+        'heard IS NOT NULL',
+        "$arg{value} IS NOT NULL",
+    );
+    my @bind;
+
+    if ($arg{receiver} ne '') {
+        push @where, 'receiver = ?';
+        push @bind, $arg{receiver};
+    }
+
+    if ($arg{heard} ne '') {
+        push @where, 'heard = ?';
+        push @bind, $arg{heard};
+    }
+
+    if (@{ $arg{pairs} }) {
+        my @pair_terms;
+        for my $p (@{ $arg{pairs} }) {
+            push @pair_terms, '(receiver = ? AND heard = ?)';
+            push @bind, $p->{receiver}, $p->{heard};
+        }
+        push @where, '(' . join(' OR ', @pair_terms) . ')';
+    }
+
+    if (defined $arg{gps_max} && $arg{gps_max} ne '') {
+        push @where, 'gps_fix_age_ms <= ?';
+        push @bind, int($arg{gps_max});
+    }
+
+    if (defined $arg{gps_valid} && $arg{gps_valid} ne '') {
+        push @where, 'gps_valid = ?';
+        push @bind, int($arg{gps_valid});
+    }
+
+    if (defined $arg{clock_valid} && $arg{clock_valid} ne '') {
+        push @where, 'clock_valid = ?';
+        push @bind, int($arg{clock_valid});
+    }
+
+    my $sql = qq{
+        SELECT
+            COUNT(*) AS n_rows,
+            MIN(rx_epoch_ms) AS first_rx_epoch_ms,
+            MAX(rx_epoch_ms) AS last_rx_epoch_ms,
+            MIN($arg{value}) AS min_plot_rssi,
+            AVG($arg{value}) AS avg_plot_rssi,
+            MAX($arg{value}) AS max_plot_rssi,
+            MIN(vbat_mv) AS min_vbat_mv,
+            MAX(vbat_mv) AS max_vbat_mv,
+            MAX(gps_fix_age_ms) AS max_gps_fix_age_ms
+        FROM ble_observation
+        WHERE } . join(' AND ', @where);
+
+    my $r = $dbh->selectrow_hashref($sql, undef, @bind) || {};
+    $r->{db_file} = $DB_FILE;
+    $r->{value} = $arg{value};
+    $r->{bucket_s} = $arg{bucket_s};
+    $r->{receiver_filter} = $arg{receiver};
+    $r->{heard_filter} = $arg{heard};
+    $r->{pairs_filter} = join(',', map { $_->{receiver} . ':' . $_->{heard} } @{ $arg{pairs} });
+    $r->{gps_max_ms_filter} = defined $arg{gps_max} ? $arg{gps_max} : '';
+    $r->{gps_valid_filter} = defined $arg{gps_valid} ? $arg{gps_valid} : '';
+    $r->{clock_valid_filter} = defined $arg{clock_valid} ? $arg{clock_valid} : '';
+
+    return $r;
+}
+
+sub parse_pairs {
+    my ($pairs_text) = @_;
+    return () unless defined $pairs_text && $pairs_text ne '';
+
+    my @pairs;
+    for my $pair (split /,/, $pairs_text) {
+        next if $pair eq '';
+        my ($receiver, $heard) = split /:/, $pair, 2;
+        $receiver //= '';
+        $heard    //= '';
+        $receiver =~ s/^\s+|\s+$//g;
+        $heard    =~ s/^\s+|\s+$//g;
+
+        if (!$VALID_UNIT{$receiver} || !$VALID_UNIT{$heard}) {
+            emit_error(400, "Bad pairs value: $pair");
+        }
+
+        push @pairs, {
+            receiver => $receiver,
+            heard    => $heard,
+        };
+    }
+
+    return @pairs;
+}
+
+sub emit_html {
+    my ($traces, $summary) = @_;
+
+    my $json = JSON::PP->new->canonical(1)->encode({
+        traces  => $traces,
+        summary => $summary,
+    });
+
+    my $title = 'Exercise 26 BLE RSSI vs Time';
+
+    print "Content-Type: text/html; charset=utf-8\r\n";
+    print "Cache-Control: no-cache\r\n";
+    print "\r\n";
+
+    print <<"HTML";
+<!DOCTYPE html>
+<!--
+  Generated by exercise26_ble_rssi_plotly.pl
+  20260526 ChatGPT
+  \$Id\$
+  \$HeadURL\$
+-->
+<html lang="en">
+<head>
+<meta charset="utf-8">
+<title>$title</title>
+<script src="$PLOTLY_JS_URL"></script>
+<style>
+  body {
+    font-family: sans-serif;
+    margin: 0;
+    padding: 0;
+  }
+  header {
+    padding: 0.75rem 1rem;
+    border-bottom: 1px solid #ccc;
+  }
+  h1 {
+    font-size: 1.2rem;
+    margin: 0 0 0.35rem 0;
+  }
+  .meta {
+    font-size: 0.85rem;
+    line-height: 1.35;
+  }
+  #chart {
+    width: 100vw;
+    height: calc(100vh - 7rem);
+  }
+  code {
+    white-space: nowrap;
+  }
+</style>
+</head>
+<body>
+<header>
+  <h1>$title</h1>
+  <div class="meta" id="summary"></div>
+</header>
+<div id="chart"></div>
+<script>
+const DATA = $json;
+
+function msToDateArray(xs) {
+  return xs.map(ms => new Date(ms));
+}
+
+const traces = DATA.traces.map(t => ({
+  type: t.type,
+  mode: t.mode,
+  name: t.name,
+  x: msToDateArray(t.x_ms),
+  y: t.y,
+  customdata: t.customdata,
+  hovertemplate: t.hovertemplate,
+  connectgaps: false
+}));
+
+const s = DATA.summary;
+document.getElementById('summary').innerHTML = [
+  '<b>Database:</b> <code>' + escapeHtml(s.db_file || '') + '</code>',
+  '<b>Rows:</b> ' + (s.n_rows || 0),
+  '<b>Y value:</b> ' + escapeHtml(s.value || ''),
+  '<b>Bucket seconds:</b> ' + (s.bucket_s || 0),
+  '<b>Receiver:</b> ' + escapeHtml(s.receiver_filter || 'all'),
+  '<b>Heard:</b> ' + escapeHtml(s.heard_filter || 'all'),
+  '<b>Pairs:</b> ' + escapeHtml(s.pairs_filter || 'auto'),
+  '<b>Filters:</b> gps_max_ms=' + escapeHtml(s.gps_max_ms_filter || 'none') +
+    ' gps_valid=' + escapeHtml(s.gps_valid_filter || 'any') +
+    ' clock_valid=' + escapeHtml(s.clock_valid_filter || 'any')
+].join(' &nbsp; ');
+
+const layout = {
+  title: {
+    text: 'BLE RSSI over time',
+    x: 0.02
+  },
+  xaxis: {
+    title: 'Receiver observation time',
+    type: 'date'
+  },
+  yaxis: {
+    title: 'RSSI dBm. Higher, less negative, is stronger.',
+    autorange: true
+  },
+  hovermode: 'closest',
+  legend: {
+    orientation: 'h',
+    y: -0.22
+  },
+  margin: {
+    l: 70,
+    r: 30,
+    t: 50,
+    b: 90
+  }
+};
+
+const config = {
+  responsive: true,
+  displaylogo: false,
+  toImageButtonOptions: {
+    filename: 'exercise26_ble_rssi_' + timestampForFilename(),
+    format: 'png',
+    width: 1600,
+    height: 900,
+    scale: 1
+  }
+};
+
+Plotly.newPlot('chart', traces, layout, config);
+
+function escapeHtml(s) {
+  return String(s)
+    .replace(/&/g, '&amp;')
+    .replace(/</g, '&lt;')
+    .replace(/>/g, '&gt;')
+    .replace(/"/g, '&quot;')
+    .replace(/'/g, '&#39;');
+}
+
+function timestampForFilename() {
+  const d = new Date();
+  const pad = n => String(n).padStart(2, '0');
+  return d.getFullYear() + pad(d.getMonth() + 1) + pad(d.getDate()) + '_' +
+    pad(d.getHours()) + pad(d.getMinutes()) + pad(d.getSeconds());
+}
+</script>
+</body>
+</html>
+HTML
+}
+
+sub parse_query_string {
+    my $qs = $ENV{QUERY_STRING} // '';
+
+    # Console convenience:
+    #   ./exercise26_ble_rssi_plotly.pl 'receiver=BOB&value=avg_rssi'
+    #   ./exercise26_ble_rssi_plotly.pl receiver=BOB value=avg_rssi
+    # CGI still uses QUERY_STRING.
+    if ($qs eq '' && @ARGV) {
+        if (@ARGV == 1 && $ARGV[0] =~ /[&=]/) {
+            $qs = $ARGV[0];
+        }
+        else {
+            $qs = join('&', @ARGV);
+        }
+    }
+
+    my %out;
+
+    for my $pair (split /&/, $qs) {
+        next if $pair eq '';
+
+        my ($k, $v) = split /=/, $pair, 2;
+        $k = url_decode($k // '');
+        $v = url_decode($v // '');
+
+        next if $k eq '';
+
+        $out{$k} = $v;
+    }
+
+    return %out;
+}
+
+sub url_decode {
+    my ($s) = @_;
+
+    $s =~ tr/+/ /;
+    $s =~ s/%([0-9A-Fa-f]{2})/chr(hex($1))/eg;
+
+    return $s;
+}
+
+sub numeric {
+    my ($v) = @_;
+
+    return undef unless defined $v;
+    return 0 + $v;
+}
+
+sub emit_error {
+    my ($status, $msg) = @_;
+
+    my %status_text = (
+        400 => 'Bad Request',
+        500 => 'Internal Server Error',
+    );
+
+    my $text = $status_text{$status} // 'Error';
+
+    print "Status: $status $text\r\n";
+    print "Content-Type: application/json\r\n";
+    print "Cache-Control: no-cache\r\n";
+    print "\r\n";
+    print encode_json({
+        error  => $text,
+        status => $status,
+        detail => $msg,
+    });
+    print "\n";
+
+    exit 0;
+}