Once upon a time, there was the original R Graph Gallery, by Romain François. Sadly, it's been unavailable for several years. Now there's a new R Graph Gallery to fill the void, created by Yan Holtz. It contains more than 200 data visualizations categorized by type, along with the R code that created them.
You can browse the gallery by types of chart (boxplots, maps, histograms, interactive charts, 3-D charts, etc), or search the chart descriptions. Once you've found a chart you like, you can admire it in the gallery (and interact with it, if possible), and also find the R code which you can adapt for your own use. Some entries even include mini-tutorials describing how the chart was made. You can even submit your own graph, if you'd like to have it displayed in the gallery as well.
It also includes a number of graphics that are not data visualizations, but rather Data Art: artistic creations based on data and/or code, including this neat rendering of BB-8 in R code. Sadly it doesn't include any examples from the Accidental aRt collection, nor this new Dadaism from Stephen Turner.
If you're looking for some R code to adapt for a visualization, or just looking for inspiration, take a look at the R Graph Gallery.