Stacked graphs in /graph have increasingly incorrect max values

[smarterclayton]

Bug Report

Generated a stacked graph that was many small aggregates over a long window (a week). As I increased the window size from 1 hour to 1 week, the max Y value of the graph was increasingly inaccurate, and the stacked data went from filling 90% of the min -> max Y value to filling only 5-10% of the vertical area of the graph. There were no outlier values that I am aware of above that limit. If I disable stacking on the long window, the series occupy most of the range of the vertical area.

I expected as I increased the window that stacked would continue to take up most of the Y range.

It looks as if it is estimating the max Y incorrectly the larger the range.

Environment

• Prometheus version:

2.3.2
71af5e29e815795e9dd14742ee7725682fa14b7b
20180712-14:02:52
go1.10.3