Add zoom rectangles to plotting tool

This makes it so that right-click + drag on the plots draws a rectangle
which we will zoom to.

Change-Id: I968f56151228831b6f54d9f850dc6790e4a79052
diff --git a/frc971/analysis/plot_index.ts b/frc971/analysis/plot_index.ts
index 759202a..8fbdb7c 100644
--- a/frc971/analysis/plot_index.ts
+++ b/frc971/analysis/plot_index.ts
@@ -34,7 +34,8 @@
 const helpDiv = document.createElement('div');
 rootDiv.appendChild(helpDiv);
 helpDiv.innerHTML =
-    'Help: click + drag to pan, double click to reset, scroll to zoom. ' +
+    'Help: click + drag to pan, double click to reset, scroll to zoom, ' +
+    'right-click + drag to zoom to rectangle, Esc to cancel. ' +
     'Hold the x/y keys to only pan/zoom along the x/y axes.';
 
 class PlotState {