Explain difference between Flame Graph and Stack Chart

README.md

@@ -35,6 +35,9 @@ Vernier.trace(out: "time_profile.json") { some_slow_method }
 
 The output can then be viewed in the Firefox Profiler (demo) or the [`profile-viewer` gem](https://github.com/tenderlove/profiler/tree/ruby) (a Ruby-customized version of the firefox profiler.
 
+- **Flame Graph**: Shows proportionally how much time is spent within particular stack frames. Frames are grouped together, which means that x-axis / left-to-right order is not meaningful.
+- **Stack Chart**: Shows the stack at each sample with the x-axis representing time and can be read left-to-right.
+
 ### Retained memory
 
 Record a flamegraph of all **retained** allocations from loading `irb`.