RFR: 7120: Charts in custom pages are not auto refreshed

[Alex Macdonald]

This PR addresses JMC-7120 [0], in which charts in the custom pages are not auto refreshed. This issue is a regression from the changes to the `ChartCanvas` in the threads page enhancement, sorry about that.

The culprit here was the wrongful removal of a `redrawChart()` call in `ChartCanvas.setChart`. Looking back through the old commits I have for the Threads Page work I couldn't find where it was removed, but it must have been pretty early into the work. I noticed that the `redrawChart()` is present in JMC 7 [1] and in the commit prior to the threads page pr going in [2], but not afterwards [3]. This PR simply adds back the redraw call.

gif:
![7120](https://user-images.githubusercontent.com/10425301/106956199-462fcd00-6704-11eb-9dad-f60ff55590a5.gif)

[0] https://bugs.openjdk.java.net/browse/JMC-7120
[1] https://hg.openjdk.java.net/jmc/jmc7/file/63ec7d0ee8d9/application/org.openjdk.jmc.ui/src/main/java/org/openjdk/jmc/ui/misc/ChartCanvas.java#l533
[2] https://github.com/openjdk/jmc/blob/c3f49204ce36139ac83ccfa88b2ada26a0b2cd04/application/org.openjdk.jmc.ui/src/main/java/org/openjdk/jmc/ui/misc/ChartCanvas.java#L533
[3] https://github.com/openjdk/jmc/blob/5feaf25fa2a4a61236a733053dc60134c89efccf/application/org.openjdk.jmc.ui/src/main/java/org/openjdk/jmc/ui/misc/ChartCanvas.java#L716

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Commit messages:
 - 7120: Charts in custom pages are not auto refreshed

Changes: https://git.openjdk.java.net/jmc/pull/211/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=211&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7120
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/211.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/211/head:pull/211

PR: https://git.openjdk.java.net/jmc/pull/211