Consistent NA handling

[wch]

We need to add consistent handling for NA values.

[hadley]

For now, we should probably just take the ggplot2 approach - i.e. drop them, with a warning.

[kaelte]

When will the NA values been dropped?

The value (NA,NA) will be plotted at the minimal x-value and maximal y-value. That adds a point in the top left corner in case of layer_points() but a line across the plot in case of layer_lines() and the (NA,NA) value being the last value.