[client] imgui: make graph y-axis configurable

The default of [0, 50] makes sense for FPS/UPS graphs, but does not for
things like the import graph. The latter should not take more than 5 ms
for sure.

This commit allows the min/max y-axis value to be specified when registering
the graph.

client/include/app.h

@@ -98,7 +98,7 @@ void app_freeOverlays(void);
 struct OverlayGraph;
 typedef struct OverlayGraph * GraphHandle;
 
-GraphHandle app_registerGraph(const char * name, RingBuffer buffer);
+GraphHandle app_registerGraph(const char * name, RingBuffer buffer, float min, float max);
 void app_unregisterGraph(GraphHandle handle);
 
 void app_clipboardRelease(void);