How to create a custom geom?

[hebertodelrio]

I am trying to create a custom geom, but I do not seem to figure out how to do it. For example, the following R code creates a geom which allows to draw two parallel linear models. This cannot be accomplished with geom_smooth(aes(x='some x data', y='some y data', hue='some hue data')), since this will create two linear models with y-intercept depending on the hue data, as well as the slope.

Any help is welcomed.

geom_parallel_slopes.txt