Repository: google/pprof (language: go)

Issue:
Why cum has negative values？
![image](https://user-images.githubusercontent.com/8255607/140287225-4a6e52b0-cc98-4c67-b0cd-ebdf7d2962bf.png)

It saids negative cum values turns green.But i cannont understand,why cum value could be negative ?


Discussion / hints:
Negative values (cumulative or self values) are most likely to occur when one is doing profile comparison, which the pprof CLI does by subtracting one profile from another. More details on profile comparison can be found in pprof's docs at https://github.com/google/pprof/tree/master/doc#comparing-profiles. 

I'll send a PR to note this. 



Fix patch:
diff --git a/doc/README.md b/doc/README.md
index 0eb7b7668..9144b004e 100644
--- a/doc/README.md
+++ b/doc/README.md
@@ -192,7 +192,9 @@ the graph, controlled by the *nodecount* option.
 
 * **Node Color**:
   * large positive cum values are red.
-  * large negative cum values are green.
+  * large negative cum values are green; negative values are most likely to
+    appear during profile comparison, see [this section](#comparing-profiles)
+    for details.
   * cum values close to zero are grey.
 
 * **Node Font Size**:


Test patch: