RFR: 6832: Should allow plotting line charts for all quantities

[Aymane Harmaz]

This PR makes sure line charts cover all the events across multiple IItemIterables, and makes it possible for linear quantities to be represented as line charts on custom pages.

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Commit messages:
 - 6832: Should allow plotting line charts for all quantities

Changes: https://git.openjdk.org/jmc/pull/688/files
  Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=688&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-6832
  Stats: 34 lines in 3 files changed: 3 ins; 13 del; 18 mod
  Patch: https://git.openjdk.org/jmc/pull/688.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/688/head:pull/688

PR: https://git.openjdk.org/jmc/pull/688