flamegraph: Retain inversion when toggling filters

Previously clicking the "Hide Irrelevant Frames" or "Hide Import System
Frames" checkboxes would switch the flame graph back to icicle mode.
Ensure we retain flame mode when the user has chosen it.

news/656.bugfix.rst

@@ -0,0 +1,3 @@
+Ensure flame graphs stay in flame mode when the user has selected it.
+Previously clicking the "Hide Irrelevant Frames" or "Hide Import System Frames"
+checkboxes would switch the flame graph back to icicle mode.

src/memray/reporters/assets/flamegraph_common.js

@@ -258,6 +258,9 @@ export function initThreadsDropdown(data, merge_threads) {
 }
 
 export function drawChart(chart_data) {
+  // Retain the "invertedness" if there's an existing graph.
+  let invert = chart ? chart.inverted() : true;
+
   // Clear any existing chart
   if (chart) {
     chart.destroy();
@@ -270,8 +273,7 @@ export function drawChart(chart_data) {
     // smooth transitions
     .transitionDuration(250)
     .transitionEase(d3.easeCubic)
-    // invert the graph by default
-    .inverted(true)
+    .inverted(invert)
     // make each row a little taller
     .cellHeight(20)
     // don't show elements that are less than 5px wide