power: Fix rounding error.

BUG=990870
TEST=copy power.js locally to chromeOS device exhibiting failure and
no longer see 'undefined'

Change-Id: I1d79164bdae6e4bf239a7da5581e9f37c813e6f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1777103
Auto-Submit: Todd Broch <tbroch@chromium.org>
Reviewed-by: Satoru Takabayashi <satorux@chromium.org>
Commit-Queue: Todd Broch <tbroch@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692096}

chrome/browser/resources/chromeos/power.js

@@ -376,7 +376,7 @@ function plotLineGraph(
     }
 
     if (width == size) {
-      drawTimeGuide(x - xOrigin);
+      drawTimeGuide(Math.round(x - xOrigin));
     } else {
       drawTimeGuide(Math.round((x - xOrigin) / (width - 1) * (size - 1)));
     }