[R-sig-teaching] Choice of graphics package

[Randall Pruim]

On Nov 27, 2015, at 3:37 PM, Ista Zahn <istazahn at gmail.com<mailto:istazahn at gmail.com>> wrote:


> (Theming in ggplot2 generally requires you to do something to each plot, one of the downsides of ggplot2.)

That is completely untrue. Whatever downsides ggplot2 may have, this is not one of them.

Ista,

My guess is that you didn’t understand my claim.  Either that or you know something about ggplot2 that I’ve not been able to locate anywhere.  (Since Hadley is on  this list, he can chime in if I’m missing something.)

So let me explain what I mean with an example.

Suppose I want to create six plots of various sorts.  Each of them uses color according to a factor a with three levels.  That's groups = a in lattice and colour = a in ggplot2.  So far so good.

But after I have created all of these plots with the default color choices, I decide I want to have the colors be blue, red, and 50% gray.  In lattice, I can put this information into the default theme with

trellis.par.set( superpose.symbol = list(col = c("blue", "red", "gray50")) )

and without any adjustments to any of the plotting code, all the plots will update to the new color scheme.   show.settings() will even show me what my default theme looks like.

One line of code changes in one place and all the plots are using the new color scheme.  And if my new choices are choices I will use frequently, I can put the theme into a package and do something like

trellis.par.set(theme = theme.mosaic())

There is also the option to add these themes to individual plots using the par.settings argument — this is more like the ggplot2 way.

In ggplot2, I need to add on scale_colour_manual() to EACH PLOT, or write some sort of wrapper that does the job for me.  In either case, if I started out using the defaults and want to make document-wide changes, I need to edit EACH plot to get the desired affect.  In lattice, this is one line of code and all the plots are good to go.

Last I looked for this, ggplot2 did not provide a way to set these sorts of defaults and generally prefers a system where all of this sort of theming is located local to each plot.  So plots end up having a bunch of theme stuff added on to them like

+ theme_minimal()
+ scale_colour_manual(values = c(“blue”, “red”, “gray50”)
+ etc.

See the examples at

http://docs.ggplot2.org/dev/vignettes/themes.html

Especially for teaching, I find this to be a real problem.  I don’t want to clutter up early examples with all this sort of formatting, but I do want to control the choice of colors used so that they work well for printing or projecting in my local environment.  When I’m producing a journal or some other sort of report, this is less important because I generally don’t show the code in the final document — only the plot.  But it still violates the DRY principle (Don’t Repeat Yourself).

—rjp

PS.  I should have mentioned one commonly held, but poorly justified, reason to prefer ggplot2 over lattice:  the default colors.  lattice makes it easy to adjust the defaults.  latticeExtra even provides a ggplot2 look-alike theme that will make your lattice plots look a lot like ggplot2 plots.  (The won’t match exactly because they don’t include all of the same elements.  Liking elements available in one system but not in the other is a possible reason to prefer one over the other.)  On the other hand, if you don’t like the defaults in ggplot2 — you’re out of luck, you don’t get the change the defaults.





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