Add setting for grid spacing on temporal axis (EmbarkStudios#195)

Add the ability to change the spacing of the x-axis indicators from the
default 10ms to an arbitrary interval. This is particularly useful to
determine when a frame misses its desired vblank.

---

This change is primarily motivated by my own necessity to easily
determine when a particular frame ran for over ~16ms and specifically
which part of the flamegraph caused it to stretch beyond that limit.
It's probably not the ideal solution, but I wanted to open up a PR to
see if there's any interest in this feature from upstream.

One particularity of the current implementation is that 1, 10, 100, and
1000 all behave the same, since the interval is somewhat relative to the
current viewport's width. I don't care about this since it doesn't
really make a difference (you just need to zoom in/out), but I can see
how that might be an undesired user experience.


https://github.com/EmbarkStudios/puffin/assets/8886672/6277f290-f93b-4603-bd58-3abd4c54d41c

---------

Co-authored-by: Emil Ernerfeldt <[email protected]>

puffin_egui/src/flamegraph.rs

@@ -124,6 +124,9 @@ pub struct Options {
     /// Visual settings for threads.
     pub flamegraph_threads: IndexMap<String, ThreadVisualizationSettings>,
 
+    /// Interval of vertical timeline indicators.
+    grid_spacing_micros: f64,
+
     #[cfg_attr(feature = "serde", serde(skip))]
     filter: Filter,
 
@@ -148,10 +151,12 @@ impl Default for Options {
             rounding: 4.0,
 
             frame_list_height: 48.0,
-            frame_width: 10.0,
+            frame_width: 10.,
 
             merge_scopes: false, // off, because it really only works well for single-threaded profiling
 
+            grid_spacing_micros: 1.,
+
             sorting: Default::default(),
             filter: Default::default(),
 
@@ -256,6 +261,16 @@ pub fn ui(
             });
 
             options.filter.ui(ui);
+
+            // Grid spacing interval selector.
+            ui.horizontal(|ui| {
+                ui.label("Grid spacing:");
+                let grid_spacing_drag = DragValue::new(&mut options.grid_spacing_micros)
+                    .speed(0.1)
+                    .clamp_range(1.0..=100.0)
+                    .suffix(" µs");
+                grid_spacing_drag.ui(ui);
+            });
         });
 
         ui.collapsing("Visible Threads", |ui| {
@@ -505,7 +520,7 @@ fn paint_timeline(
     // We show all measurements relative to start_ns
 
     let max_lines = canvas.width() / 4.0;
-    let mut grid_spacing_ns = 1_000;
+    let mut grid_spacing_ns = (options.grid_spacing_micros * 1_000.) as i64;
     while options.canvas_width_ns / (grid_spacing_ns as f32) > max_lines {
         grid_spacing_ns *= 10;
     }