I was inspired by a Google+ conversation about this figure. So I took numbers of executions from here and population from here (28 January 2013). Execution numbers are from 2011 and population, I think from 2008 to 2011. Malaysia and Syria were not included as no number was given in the source for the number of executions. For China I rounded the number to 1000 even though the source gives an estimate from about 1000 to 4000. The original spreadsheet can be found here
Figure 3. Histogram of execution densities.
Table 1. Data the images are based on.
Edit: Added Figure 3.