improve pat_scatterplot()

[jonathancallahan]

The pat_scatterplot() is a good working example but can be improved.

If it's not too hard, we should convert it to ggplot graphics.

Desirable features:

• parameters argument -- We should allow people to pass in a vector of the parameters they want to generate scatterplots for. Any combination of pat internal variables should be OK. But we also want to default to datetime, A, B, temp, humidity.
• graphical parameters optimized for speed -- shape = 18? or '.'
• sampleFraction argument -- For large data frames we will optimize things by randomly sampling the data. The sampleFraction argument should default to NULL which means we subsample to get a data frame with a reasonable number of rows, say 1e5. But users will be allowed to set this parameter to 1 if they want to force it to plot everything or a low number if they want to speed things up
• ... argument -- We should allow people to supply extra graphical parameters to customize the plot like color, size, shape, etc.