Area Chart doesn't fill in area when a new category is added

[etr2460]

If you have an area chart where a series starts part way through the time range, the chart won't fill in properly.

Expected results

No blank spaces

Actual results

Blank spaces

Screenshots

How to reproduce the bug

Make an area chart with a series that starts half way through the time range

Environment

• superset version: master
• python version: 3.6
• node.js version: 8.6

Checklist

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• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

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