Area chart fill in subzero values when null in data

[tfilo]

Description

When rendering area type chart, and there are nulls in data in subzero/low values, than fill will occurs from zero value instead of max value. There is some kind of issue when null occur in high parts of graph too, but it's not so obvious as with low values.

Steps to Reproduce

1. create area chart with values going from high to low and back to high
2. introduce some nulls in data in low values
3. fills will occurs between nulls

Expected Behavior

I would expect that it will looks like chart without nulls, fill should be calculated from max value to lowest value regardless of missing (null) data.

Actual Behavior

In place of null data it starts to render fills from zero.

Screenshots

Reproduction Link

Here is simple HTML file with two charts, second one has some nulls introduced.
apex.html