Saving charts • sgplot

This article covers best practice guidance for saving charts as images. This may be as a standalone image file, or embedded in a document produced using R Markdown.

This guidance follows recommendations in the Government Analysis Function charts guidance.

In summary, the following good practice is recommended:

Save charts in SVG image format.
Size a chart to be roughly the same width as the content surrounding it.
For landscape charts on an A4 page, 159 mm x 100 mm are good dimensions.
Remember to include alt text when including charts in an R Markdown document.

File format

The best format to save images of charts is Scalable Vector Graphic (SVG) format. SVG files retain the same quality regardless of the size they are viewed at. This means SVG images are more accessible than other image formats such as JPG or PNG.

You can find an example of a chart image in an SVG, JPEG and PNG format on GOV.UK.

If you cannot use an SVG image, then a JPG or PNG image is acceptable.

Size and dimensions

A chart should follow the flow of the content surrounding it. It should be roughly the same width as the text before and after the image.

The dimensions of your chart will depend on where you plan to use it. For example, if you’re using a page-based document such as Microsoft Word, and would like to include a landscape image on an A4 portrait page, a width of 159 mm and height of 100 mm works well.

When using the SVG file format, actual saved image dimensions will be exactly as specified. However, when using JPG or PNG formats, they may not be. This is because these file formats are also influenced by DPI (dots per inch). DPI is a measure of the resolution of an image; the higher the DPI, the more crisp and good quality your image will be. This is another good reason to use SVG over these formats, however if you are limited to JPG or PNG, it may take some trial and error to find the correct combination of width, height and DPI.

Examples

Standalone image file

The ggsave() function from ggplot2 can be used to save a standalone image file. To specify the SVG file format, simply ensure your file name ends with .svg. The following example uses the recommended dimensions from the previous section.

ggsave(myplot, filename = "example-plot.svg",
       width = 159, height = 100, units = "mm")

For a JPG or PNG image, also consider the DPI value. In ggsave, this can be supplied as a numerical value, or as a string input. For best quality, use “retina”.

ggsave(myplot, filename = "example-plot.png",
       width = 159, height = 100, units = "mm", dpi = "retina")

R Markdown

For charts in an R Markdown document, file format, dimensions and other settings can be set in the code chunk options.

Format: Set dev = "svg", otherwise the default format of png will be used.
Dimensions: Set fig.height and fig.width. For HTML outputs, the default figure dimensions are often suitable, however when knitting to Microsoft Word, the default dimensions often result in a small image and so it is best to specify these in the R Markdown code chunk. Note that R Markdown requires figure dimensions in inches. If you’d rather use millimetres, use the mm_to_inch helper function from sgplot.
Alt Text: For HTML outputs, use fig.alt, and for Microsoft Word outputs, fig.cap. Note that by default, the value given to fig.cap will be shown under the chart. If you’d rather turn this off, you can control this in the R Markdown YAML header:
```
output: 
  word_document:
   fig_caption: no
```

Set these options in a code chunk header:

```{r plot1, dev = "svg", fig.width = mm_to_inch(159), fig.height = mm_to_inch(100), fig.alt = "A description of plot1"}
plot1
```

It can sometimes make your code easier to read by specifying chunk options within the chunk:

```{r plot1}
#| dev = "svg"
#| fig.width = mm_to_inch(159)
#| fig.height = mm_to_inch(100)
#| fig.alt = "A description of plot1"

plot1
```

If you would like all of your images to be in the same format, it is usually easier to set options globally in the first code chunk of your document:

```{r setup, include = FALSE}
knitr::opts_chunk$set(
  dev = "svg",
  fig.width = mm_to_inch(159), 
  fig.height = mm_to_inch(100)
)
```

Note that sgplot must be loaded before setting global options, otherwise you will need to specify the package when calling mm_to_inch; e.g. sgplot::mm_to_inch().

There may be cases where you’d like all charts to be in SVG format, but with different dimensions. In this case you can set dev = "svg" in the global options and fig.width and fig.height for each code chunk separately.

Knitting to Microsoft Word

You may receive a warning message referring to rsvg-convert when knitting an R Markdown document containing SVG images to Microsoft Word. This can be safely ignored.
It is not possible to centre charts or images when knitting to Microsoft Word. A workaround for this is to use a Word style template document.