Allow for customization of showAreaAndBarPlots and showLinePlots

[fbenke-pik]

Motivation

Sometimes, we want to add customization to the objects returned by the show functions showAreaAndBarPlots and showLinePlots. This is why the creation and rendering of plots was split into separate functions (#138). For createAreaAndBarPlots and createLinePlots, the returned objects are gtable objects, which are still hard to manipulate.

Changes

This PR introduces intermediate steps layoutLinePlots and layoutAreaAndBarPlots, which take care of layouting steps previously done in the create function. The create functions return a list of ggplot elements. The layout functions accept the list returned by the create function, applies layouting and returns gtable objects. The show function calls the layout function (return value is identical to previous return of the create function) and renders the result.

Other plot formats currently don't need an additional layout step, as the create functions already return a list of ggplot objects.

Usage

This PR allows for customization of ggplot objects by calling the create function in your own plotting function or in your markdown. One can make adjustments to the returned items (while maintaining the original list structure) and then can pass the modified list to the layout function.

Example

myitems <- createAreaAndBarPlots(...)

# ...
# modify myitems
# ...

showPlot(layoutAreaAndBarPlots(myitems))[1])