Adding a scale to an image plot

August 31, 2011

(This article was first published on me nugget, and kindly contributed to R-bloggers)

Here's a function that allows you to add a color scale legend to an image plot (or probably any plot needing a z-level scale). I found myself having to program this over and over again, and just decided to make a plotting function for future use. While I really like the look of levelplot(), the modular aspect of image() makes it much more handy to combine with other plotting commands or overlays.
For example, as far as I can tell, the simple addition of the triangle symbol to mark the highest point in the above map of Maunga Whau volcano is not possible with levelplot.
After adding this symbol, the function below – image.scale() – was used to add the accompanying color scale to another area of the device.

The function…

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