mtplot: option to render all points with a single pressure This is useful for looking at linearity when pressure flucturates a lot. BUG=chromium:358292 TEST=tested on Pixel Change-Id: I0f32b82de297623c5d7deb043f74da755d8fa7f6 Reviewed-on: https://chromium-review.googlesource.com/192370 Reviewed-by: Yufeng Shen <[email protected]> Commit-Queue: Andrew de los Reyes <[email protected]> Tested-by: Andrew de los Reyes <[email protected]>