[vis] x label is not shown

[yileic]

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• I have checked the superset logs for python stacktraces and included it here as text if 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

Superset version

latest master branch

Expected results

In line chart, when x label is entered, the x label should be shown in the chart

Actual results

The x label is not shown, neither instantly nor after re-querying, as the following picture shows:

Steps to reproduce

[yileic]

Caused by the code commented as "// Hack to adjust margins to accommodate long axis tick labels." in nvd3_vis.js.
The bottom margin in that part of code does not have room for the x axis text label.
@ascott @mistercrunch