I currently work on a HTML5/JavaScript based viewer for CSV files generated by rtl_power. It renderes the CSV files locally on the client and doesn't rely on a server in any way.
Also it's open-source

It would be great if anybody finds the time for testing it and giving me feedback on it

If you find any bugs, please let me know.
You can find the demo page at: http://heat.wq.lc
The sources are at: https://github.com/donothingloop/rtl_heatmap/
Thanks,
donothingloop