add plot into layout placeholder

phl_with_plot(x, olay, id, plotFnc, ...)

Arguments

x	rpptx object
olay	an OfficerLayout object created using `phl_layout`
id	an single integer with an id of the placeholder from `olay` object.
plotFnc	a function which creates a plot when called.
...	other arguments passed to `png` function.