Adds plotutil example image width/height unit

[zyluo]

Please take a look.

[sbinet]

@kortschak ah, I didn't notice. I guess another change in the X/wayland stack that ripples on vg/vggio...

[zyluo]

I guess this patch is gated by vggio?

[kortschak]

If you rebase this onto master, it should pass.

[zyluo]

rebased to master

[codecov-commenter]

Codecov Report

Base: 73.34% // Head: 74.77% // Increases project coverage by +1.42% :tada:

Coverage data is based on head (fafe8fe) compared to base (f21f923). Patch has no changes to coverable lines.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #756 +/- ## ========================================== + Coverage 73.34% 74.77% +1.42% ========================================== Files 56 58 +2 Lines 5159 7345 +2186 ========================================== + Hits 3784 5492 +1708 - Misses 1191 1657 +466 - Partials 184 196 +12 ``` | [Impacted Files](https://codecov.io/gh/gonum/plot/pull/756?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum) | Coverage Δ | | |---|---|---| | [vg/geom.go](https://codecov.io/gh/gonum/plot/pull/756?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum#diff-dmcvZ2VvbS5nbw==) | `76.00% <0.00%> (-5.25%)` | :arrow_down: | | [plotter/quartile.go](https://codecov.io/gh/gonum/plot/pull/756?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum#diff-cGxvdHRlci9xdWFydGlsZS5nbw==) | `67.97% <0.00%> (-2.12%)` | :arrow_down: | | [palette/hsva.go](https://codecov.io/gh/gonum/plot/pull/756?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum#diff-cGFsZXR0ZS9oc3ZhLmdv) | `89.74% <0.00%> (-1.79%)` | :arrow_down: | | [text/plain.go](https://codecov.io/gh/gonum/plot/pull/756?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum#diff-dGV4dC9wbGFpbi5nbw==) | `88.63% <0.00%> (-1.69%)` | :arrow_down: | | [plotter/errbars.go](https://codecov.io/gh/gonum/plot/pull/756?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum#diff-cGxvdHRlci9lcnJiYXJzLmdv) | `86.56% <0.00%> (-1.44%)` | :arrow_down: | | [vg/vgpdf/vgpdf.go](https://codecov.io/gh/gonum/plot/pull/756?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum#diff-dmcvdmdwZGYvdmdwZGYuZ28=) | `68.58% <0.00%> (-1.29%)` | :arrow_down: | | [axis.go](https://codecov.io/gh/gonum/plot/pull/756?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum#diff-YXhpcy5nbw==) | `85.96% <0.00%> (-0.99%)` | :arrow_down: | | [vg/tee.go](https://codecov.io/gh/gonum/plot/pull/756?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum#diff-dmcvdGVlLmdv) | `76.00% <0.00%> (-0.93%)` | :arrow_down: | | [palette/moreland/smooth.go](https://codecov.io/gh/gonum/plot/pull/756?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum#diff-cGFsZXR0ZS9tb3JlbGFuZC9zbW9vdGguZ28=) | `85.91% <0.00%> (-0.63%)` | :arrow_down: | | [plotter/line.go](https://codecov.io/gh/gonum/plot/pull/756?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum#diff-cGxvdHRlci9saW5lLmdv) | `90.38% <0.00%> (-0.42%)` | :arrow_down: | | ... and [45 more](https://codecov.io/gh/gonum/plot/pull/756?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum) | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=gonum)

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