Creating scatter plot by ggplot for long format data

问题

I have worked on a long format data.frame with different number of entities in each condition like this:

Condition   rating
Control 2.1596456
Control 0.2385878
Control 3.0042808
Control 5.8012919
Treatment1  5.7561241
Treatment1  0.1879127
Treatment20 0.3968469
Treatment20 3.9423182

As I find out ggplot creates scatter plot usually for wide format data not long ones.

How can I create a pair scatter plot for this data.frame? (e.g. Control-Treatment1 and Treatment1-Treatment20)

Thanks is advance.

来源：https://stackoverflow.com/questions/39314389/creating-scatter-plot-by-ggplot-for-long-format-data