We have now added an indicator of how countries are governed. This indicator puts countries on a spectrum from democracy to autocracy through expert scoring on a number of criteria. The range of the indicator is from 10 (fully instititutionalized democracy) to -10 (hereditary monarchy). See the example graph here (look at the colour of the bubbles for the democracy score). Data are available all the way from 1800 to 2007.
The Polity IV dataset is produced by a group based at the University of Maryland in the United States. Indicators of democracy are always subjective and Polity is no exception. However, this dataset is clearly documented: is is very clear what the questions the researchers have asked themselves and what their answers are to these question. The full documentation is available here.
Because these data are ordinal – meaning that we can say that a higher score means more democracy, but not necessarily that the difference between a score of 9 and 10 means the same as the difference between, say, 0 and 1 – the most appropriate way to use this indicator is for colouring the bubbles, as in the example graph above.