I've done this before by exporting the KML of the data and importing it into a Fusion Table[1] which has a built-in heatmap view. I tried to do it just now and not only have they nerfed it by only allowing you to download a 30 day range, the KML file they export doesn't seem to be readable by Fusion Tables. Disappointing.

Looks like you can still download all of your data from Google Takeout[2], though. The format is pretty easy to read, converting it into something that Fusion Tables can read should be pretty easy.

[1] http://www.google.com/drive/apps.html#fusiontables

[2] https://www.google.com/settings/takeout/custom

EDIT: I modified a json->CSV jsfiddle I found so it converts the data I got from Google Takeout to something that is readable by Fusion Tables:

http://jsfiddle.net/evan/5w9Hz/embedded/result/

However, Fusion Tables thwarted me again by only showing a heatmap of the first 1000 location records, so I'm giving up on this for now.