Increase range of the kde graph and other tweaks

[julienw]

This was bothering me for some time.

The problems:

• to compute the KDE graph, I was using the prop "min and max". The problem is that then we don't see well the contributions of the leaf most or right most points. This is more visible after #734 but was already visible before.
• when there's just one point (such as when looking at just one revision that has just one run), the graph doesn't show up, I think because the bandwidth is 0. This is visible after #734.

So this patch fixes both these issues.

Here are some before/after:

Before / After beta branch / deploy preview

Before / After beta branch / deploy preview

Before / after beta branch / deploy preview

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.50%. Comparing base (4e82cb2) to head (0279931). Report is 1 commits behind head on beta.

Files with missing lines	Patch %	Lines
src/components/CompareResults/CommonGraph.tsx	50.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## beta #740 +/- ## ========================================== - Coverage 91.58% 91.50% -0.09% ========================================== Files 84 84 Lines 2199 2201 +2 Branches 406 407 +1 ========================================== Hits 2014 2014 - Misses 162 164 +2 Partials 23 23 ```

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