webplot: add touch line chart for touch event

This patch add linechart for touch events. It plot the touch event vs
time. To use the chart, start webplot as usual, then visit /linechart.

BUG=b:62804015
TEST=Run locally

Change-Id: I6ebec0e8c22b6d8e82fedaa93518ce09c1e18356
Reviewed-on: https://chromium-review.googlesource.com/548241
Commit-Ready: Jingkui Wang <jkwang@google.com>
Tested-by: Jingkui Wang <jkwang@google.com>
Reviewed-by: Charlie Mooney <charliemooney@chromium.org>
4 files changed