vg/vgpdf: add ability to create PDFs with multiple pages
it would be great to be able to create a bunch of plots and save each in a new PDF page:
func plot(x,y []float64) (*plot.Plot, error) { ... }
func analysis() {
for i, sample := range samples {
p, err := plot(sample.x, sample.y)
// ... save p to page-i ...
}
}