RFR: 6832: Should allow plotting line charts for all quantities [v3]

[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.

Aymane Harmaz has updated the pull request incrementally with one additional commit since the last revision:

  prevented access when no more elements are available

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Changes:
  - all: https://git.openjdk.org/jmc/pull/688/files
  - new: https://git.openjdk.org/jmc/pull/688/files/4e2ff3f4..b3ecd632

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jmc&pr=688&range=02
 - incr: https://webrevs.openjdk.org/?repo=jmc&pr=688&range=01-02

  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 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