I am trying to draw a common pre-trend graph for a diff-in-diff design. My treatment group is made of one only state, while the control group is made of three states. The data are a panel so that I have obseervations for each unit (state) over the same period of time. I'll post here a portion of it

Code:
state    year    cases    pop
ACT    1971      35        151169
ACT    1972      57    159792
ACT    1973      35    173306
ACT    1974      34    186241
ACT    1975      67    199007
ACT    1976      44    207740
ACT    1977      145    213688
ACT    1978      180    217981
ACT    1979      280    220797
ACT    1980      269    224291
NSW    1971    3943    4725503
NSW    1972    3698    4795106
NSW    1973    3356    4841898
NSW    1974    3606    4894053
NSW    1975    3517    4932016
NSW    1976    3535    4959588
NSW    1977    3807    5001888
NSW    1978    4180    5053790
NSW    1979    3656    5111130
NSW    1980    3643    5171527
NSW    1981    3841    5234889
NT       1971    412    85735
NT       1972    404    92081
NT       1973    524    97127
NT       1974    563    102924
NT       1975    494    92869
NT       1976    515    98228
NT       1977    560    103938
NT       1978    719    109980
NT       1979    911    114149
NT       1980    722    118245
NT       1981    970    122616
Qld      1971    1852    1851485
Qld      1972    2039    1898478
Qld      1973    2192    1951951
Qld      1974    1952    2008340
Qld      1975    1718    2051362
Qld      1976    1492    2092375
Qld      1977    1678    2129839
Qld      1978    2107    2172047
Qld      1979    1695    2214771
Qld      1980    1838    2265935
Qld      1981    1353    2345208
Now what I would like to plot are just two lines, one for the treated group and one for the control group (three of those state in aggregate, e.g. NSW, NT and Qld). What I need to plot is the ratio "cases/pop".

What is the more meaningful way of doing it. Should I manually aggregate the data (i.e. summing cases and pop by year and create a new "control" state) or is there a easier way of doing it?