How to create a categorical bubble plot, using GNU R, similar to that used in systematic mapping studies (see below)?

categorical bubble plot used in mapping studies

EDIT: ok, here's what I've tried so far. First, my dataset (Var1 goes to the x-axis, Var2 goes to the y-axis):

> grid
                         Var1                      Var2 count
1              Does.Not.apply            Does.Not.apply    53
2               Not.specified            Does.Not.apply    15
3   Active.Learning..general.            Does.Not.apply     1
4      Problem.based.Learning            Does.Not.apply     2
5              Project.Method            Does.Not.apply     4
6         Case.based.Learning            Does.Not.apply    22
7               Peer.Learning            Does.Not.apply     6
10                      Other            Does.Not.apply     1
11             Does.Not.apply             Not.specified    15
12              Not.specified             Not.specified    15
21             Does.Not.apply Active.Learning..general.     1
23  Active.Learning..general. Active.Learning..general.     1
31             Does.Not.apply    Problem.based.Learning     2
34     Problem.based.Learning    Problem.based.Learning     2
41             Does.Not.apply            Project.Method     4
45             Project.Method            Project.Method     4
51             Does.Not.apply       Case.based.Learning    22
56        Case.based.Learning       Case.based.Learning    22
61             Does.Not.apply             Peer.Learning     6
67              Peer.Learning             Peer.Learning     6
91             Does.Not.apply                     Other     1
100                     Other                     Other     1

Then, trying to plot the data:

# Based on http://flowingdata.com/2010/11/23/how-to-make-bubble-charts/
grid <- subset(grid, count > 0)
radius <- sqrt( grid$cou
r plot
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