R stripchart() and abline(): Adding Arbitrary lines by Category

Question

Morning Community,

I was learning how to use stripchart() in R as well as the abline()function to draw arbitrary lines in the graph. I was able to draw horizontal lines when I was grouping with one variable, but was unsuccessful with two or more.

My data:

Group   Subgroup    Measure
A   1   0.234213
A   1   0.046248
A   1   0.391376
A   2   0.911849
A   2   0.729955
A   2   0.99111
A   2   0.378422
A   3   0.898037
A   3   0.258884
A   3   
A   3   0.057631
A   3   0.745202
A   3   0.121376
B   1   0.385198
B   1   0.484399
B   1   0.115034
B   1   0.073629
B   1   0.45615
B   2   0.336108
B   2   0.845458
B   2   0.267494
B   3   0.536123
B   3   1.331731
B   3   0.505114
B   3   0.843348
B   3   0.827932
B   3   0.813351
C   1   0.095587
C   1   0.158822
C   1   0.392376
C   1   0.284625
C   2   0.898819
C   2   0.743428
C   2   0.298989
C   2   0.423961
C   3   0.868351
C   3   0.181547
C   3   1.146131
C   3   0.234941

My code:

stripchart(Measure~Group*Subgroup,data=mydata,method="jitter",pch=19,jitter=0.1,col="black")
abline(h=mydata$Group)

My Results:

My Assessment: Wouldn't the abline() code apply the line to all Y-variables that has the category 'Group'? Or is it that I need to generate a new vector that combines Group and Subgroup? I have tried abline(h=mydata$Group*mydata$Subgroup) but this gives me the following error:

Warning message:
In Ops.factor(mydata$Group, mydata$Subgroup) :
  ‘*’ not meaningful for factors

In Hindsight, this wasn't going to work becuase the documentation for abline() does not allow functions. However, when I tried adding abline(h=mydata$Subgroup) The lines did not work either.