Add differential flame graph

Differential flame graphs compare two profiling runs and highlight where
performance has changed. This makes it easier to detect regressions
introduced by code changes and to verify that optimizations have the
intended effect.

The visualization renders the current profile with frame widths
representing current time consumption. Color is then applied to show the
difference relative to the baseline profile: red gradients indicate
regressions, while blue gradients indicate improvements.

Some call paths may disappear entirely between profiles. These are
referred to as elided stacks and occur when optimizations remove code
paths or when certain branches stop executing. When elided stacks are
present, an "Elided" toggle is displayed, allowing the user to switch
between the main differential view and a view showing only the removed
paths.

Author: ivonastojanovic

Doc/library/profiling.sampling.rst

@@ -1003,6 +1003,47 @@ at the top indicate functions that consume significant time either directly
 or through their callees.
 
 
+Differential flame graphs
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Differential flame graphs compare two profiling runs to highlight where
+performance changed. This helps identify regressions introduced by code changes
+and validate that optimizations achieved their intended effect::
+
+   # Capture baseline profile
+   python -m profiling.sampling run --binary -o baseline.bin script.py
+
+   # After modifying code, generate differential flamegraph
+   python -m profiling.sampling run --diff-flamegraph baseline.bin -o diff.html script.py
+
+The visualization draws the current profile with frame widths showing current
+time consumption, then applies color to indicate how each function changed
+relative to the baseline.
+
+**Color coding**:
+
+- **Red**: Functions consuming more time (regressions). Lighter shades indicate
+  modest increases, while darker shades show severe regressions.
+
+- **Blue**: Functions consuming less time (improvements). Lighter shades for
+  modest reductions, darker shades for significant speedups.
+
+- **Gray**: Minimal or no change.
+
+- **Purple**: New functions not present in the baseline.
+
+Frame colors indicate changes in **direct time** (time when the function was at
+the top of the stack, actively executing), not cumulative time including callees.
+Hovering over a frame shows comparison details including baseline time, current
+time, and the percentage change.
+
+Some call paths may disappear entirely between profiles. These are called
+**elided stacks** and occur when optimizations eliminate code paths or certain
+branches stop executing. If elided stacks are present, an elided toggle appears
+allowing you to switch between the main differential view and an elided-only
+view that shows just the removed paths (colored deep red).
+
+
 Gecko format
 ------------
 
@@ -1488,6 +1529,10 @@ Output options
 
    Generate self-contained HTML flame graph.
 
+.. option:: --diff-flamegraph <baseline.bin>
+
+   Generate differential flamegraph comparing to a baseline binary profile.
+
 .. option:: --gecko
 
    Generate Gecko JSON format for Firefox Profiler.

Lib/profiling/sampling/_flamegraph_assets/flamegraph.css

@@ -5,6 +5,29 @@
    This file extends the shared foundation with flamegraph-specific styles.
    ========================================================================== */
 
+/* --------------------------------------------------------------------------
+   Differential Flamegraph
+   -------------------------------------------------------------------------- */
+
+:root {
+  /* Regression colors */
+  --diff-regression-deep: #d32f2f;
+  --diff-regression-medium: #e57373;
+  --diff-regression-light: #ef9a9a;
+  --diff-regression-verylight: #ffcdd2;
+
+  /* Improvement colors */
+  --diff-improvement-deep: #1976d2;
+  --diff-improvement-medium: #42a5f5;
+  --diff-improvement-light: #64b5f6;
+  --diff-improvement-verylight: #90caf9;
+
+  /* Other differential colors */
+  --diff-neutral: #bdbdbd;
+  --diff-new: #9575cd;
+  --diff-elided: #d32f2f;
+}
+
 /* --------------------------------------------------------------------------
    Layout Overrides (Flamegraph-specific)
    -------------------------------------------------------------------------- */
@@ -277,7 +300,9 @@ body.resizing-sidebar {
 /* View Mode Section */
 .view-mode-section .section-content {
   display: flex;
-  justify-content: center;
+  flex-direction: column;
+  gap: 10px;
+  align-items: center;
 }
 
 /* Collapsible sections */
@@ -815,6 +840,41 @@ body.resizing-sidebar {
   color: var(--accent);
 }
 
+.tooltip-diff {
+  margin-top: 12px;
+  padding-top: 12px;
+  border-top: 1px solid var(--border);
+}
+
+.tooltip-diff-title {
+  font-size: 11px;
+  font-weight: 600;
+  color: var(--accent);
+  margin-bottom: 8px;
+}
+
+.tooltip-diff-row {
+  display: grid;
+  grid-template-columns: auto 1fr;
+  gap: 4px 14px;
+  font-size: 12px;
+  margin-bottom: 4px;
+}
+
+.tooltip-diff-row.regression .tooltip-stat-value {
+  color: rgb(220, 60, 60);
+  font-weight: 700;
+}
+
+.tooltip-diff-row.improvement .tooltip-stat-value {
+  color: rgb(60, 120, 220);
+  font-weight: 700;
+}
+
+.tooltip-diff-row.neutral .tooltip-stat-value {
+  color: var(--text-secondary);
+}
+
 .tooltip-source {
   margin-top: 10px;
   padding-top: 10px;
@@ -989,7 +1049,8 @@ body.resizing-sidebar {
    Flamegraph-Specific Toggle Override
    -------------------------------------------------------------------------- */
 
-#toggle-invert .toggle-track.on {
+#toggle-invert .toggle-track.on,
+#toggle-elided .toggle-track.on {
   background: #8e44ad;
   border-color: #8e44ad;
   box-shadow: 0 0 8px rgba(142, 68, 173, 0.3);