Our current inaccurate voting system is 6X less proportional than STV

Some people have argued that STV does not provide any system-wide guarantees of proportionality. Nonetheless, in practice, it is a highly proportional system. The plot below shows the Gallagher Index of Disproportionality for all the democratic nations with populations over 250,000 over the past 50 years. This plot shows that STV (yellow bars) delivers essentially the same level of proportionality as most of the nations that use list proportional representation systems (blue bars) and far less disproportionality (about one third as much) than the nations that use our current First Past the Post system (purple and red).

Gallagher's Disproportionality Index for 36 Democracies from 1945-1996. Data taken from an article by Prof. Arend Lijphart, Australian Democracy: Modifying Majoritarianism?, 1999. Yellow = STV, Orange = SNTV, Blue = List PR, Lavender = Alternative Vote, Purple = First Past the Post, Red = UK and Canada (Westminster-style parliament). BC has averaged a score of 20% over the past 25 years or so.