Add units to ggplot

[mailund]

Using the units package we can associate (physical) units to quantities and guarantee that we only do arithmetic on compatible units. Plotting data with associated units, however, is cumbersome in ggplot. Can we make ggplot unit-aware?

[thomasp85]

I will need some more context. What type of plot do you wish to be able to produce? Please provide some examples...

[mailund]

We have implemented unit awareness in the units package and added support for plotting them in basic plots but not with ggplot. @edzer wrote about it here: http://r-spatial.org/r/2016/09/29/plot_units.html

Essentially, we are just talking numbers with attributes that guarantee that units are converted when you multiply or divide them and that you cannot add or subtract units that can't be converted to each other. When plotting values with units we would love it if the labels would display the units together with the variables plotted.

[edzer]

@thomasp85 what context do you need? We try to reproduce the last plot in the blog post @mailund mentions with the last command, as all information is in there. ggplot gives an error message (also in the blog) that indicates a comparison to a range, but ranges are also units:

> library(units)
> with(ud_units, 1:10 * km/h)
Units: km/h
 [1]  1  2  3  4  5  6  7  8  9 10
> x = with(ud_units, 1:10 * km/h)
> range(x)
Units: km/h
[1]  1 10

or maybe this is the problem because the other argument got converted to numeric? I don't know how to use debug to enter self$oob(x, limits), where the error is triggered. @hadley mentioned we need a scale type, and I can't find documentation or examples on how to create one.

[thomasp85]

It was nicely explained in the post above - I just haven't had time to respond as I've just started on a new job...

[edzer]

Fully understandable. @mailund and I have no clue where to start with this. Maybe one of you or @hrbrmstr could give a hint?

[thomasp85]

I'll try to give it some thought this evening - stay put

[thomasp85]

I get

Warning in R_ut_init: Can't open installed, default, unit database

with both the CRAN and GitHub version on my mac every time I try to perform an operation with a unit vector...

Does that ring any bell to either of you?

[thomasp85]

Okay - I have looked into this a bit more. In terms of making sure that units are compatible: Do we agree that this is already handled since all data changes happens before being passed into ggplot? As I understand it we want to catch situations such as:

ggplot(data, aes(x = var1 + var2, y = var3))

And throw errors when var1 and var2 are not compatible... This should already work.

Now for the tough answer - we cannot make this automagical without a change to ggplot2 itself, meaning that if we want things to work after just adding e.g. geom_point() we'll need to convince @hadley to accept a change...

What we can do without any change to ggplot2 is to add a scale_[x/y]_[c/d]unit() that will automatically add the unit definition to the axis titles if the user defines the title in the constructor e.g.

ggplot(data, aes(x = var1 + var2, y = var3)) + 
  geom_point() + 
  scale_x_cunit("my axis title")

will create an x-axis title as you want... I do think we should convince @hadley to accept a change though and I'll be happy to discuss my idea with him. In essence it requires an additional method to the scale class that allows scales to modify the default label.

Please get back to me with your thoughts on this...

[thomasp85]

I've opened a PR in ggplot2 with the required changes - I recommend we wait until these are resolved...

[edzer]

Thanks for addressing this Thomas! Great to see this moving.

Regarding the warning you see: it comes from udunits2; I wrote a PR to address it in July and brought it up again a few weeks ago but got no response. @jameshiebert: how do you suggest we solve this issue? It is clearly a warning where no warning should happen.

[thomasp85]

I think it is more than an ignorable warning - when running the examples for units() I get an error about m/s not convertible to km/h which hopefully isn't intended - can you reproduce this?

[edzer]

Does it disappear when you do

library(udunits2)

first?

[thomasp85]

I'll check when I get home and get back to you

[thomasp85]

Loading udunits2 gives this message/warning

Warning in R_ut_init: Can't open installed, default, unit database
Failed to load udunits2 system database: reading shipped version from /Library/Frameworks/R.framework/Versions/3.3/Resources/library/udunits2/share/udunits2.xml

but removes the error when trying to do conversion...

[thomasp85]

I've just pushed a fork that adds rudimentary units support. Please check it out and comment there (#15)