I love this ad!

Your resport, while well hidden in the website, is quite good. Your layman term explanation on the frontend of the website is poor (and I feel is somewhat misleading). Personally, I would include a sentence or two explaining that the "Voter Power Index" (impressive name, congrats) boils down to a measure of the efficiency of the voting system in each constituency. End of point 1.
While I concede your argument is valid for a national view, at a local level (which is where your site is pitched at) is not. At present you seem (to me) to have spun it to create maximum outrage - i.e. a Lib supporter in a constituency where Tory/Lab have won could quite easily be mislead into thinking "Well, what chance did we have, my vote was worthless against all those Tory/Lab supporters" while failing to realise that their vote was just as valuable as anyone elses *within their constituency*. You don't really explicitly clarify this.

The point of PR is to remove constituency (first to the post) voting, hence your website should be making clear what the comparison actually is i.e. a localised election where you select the person you think will represent your constituency best but have less control VS a PR system where you vote nationally and have no say who then gets what once the votes are counted. Some people have issues with their local MPs but support the party as a whole, hence I feel the distinction is important.

This probably hasn't come across too well, but I plan to do a more detailed analysis of your report in time and hopefully I will be more coherent next time.