svgtools: Manipulate SVG template files of charts.

Konrad Oberwimmer konrad.oberwimmer@iqs.gv.at

Purpose

The purpose of svgtools is to manipulate SVG files that are templates of charts the user wants to produce. In vector graphics one copes with x-/y-coordinates of elements (e.g. lines, rectangles, text). Their scale is often dependent on the program that is used to produce the graphics. In applied statistics one usually has numeric values on a fixed scale (e.g. percentage values between 0 and 100) to show in a chart. Basically, svgtools transforms the statistical values into coordinates and widths/heights of the vector graphics.

SVG file format is nothing else than XML (see here). By the means of package xml2, svgtools reads SVG files and then changes certain attributes or even whole elements in the XML document.

For example, an SVG image might look like this:

Fig. 1: SVG example

Fig. 1: SVG example

Its file content contains lines in XML:

...
<g id="myBars">
    <g>
        <rect x="141.732" y="92.126" fill="#C6C6C6" width="94.394" height="14.173"/>
        <rect x="236.126" y="92.126" fill="#878787" width="94.394" height="14.173"/>
        <rect x="330.52" y="92.126" fill="#3C3C3B" width="94.677" height="14.173"/>
        <text transform="matrix(1 0 0 1 183.396 101.8799)" font-family="'ArialMT'" font-size="10">33</text>
        <text transform="matrix(1 0 0 1 277.7612 101.8799)" fill="#FFFFFF" font-family="'ArialMT'" font-size="10">33</text>
        <text transform="matrix(1 0 0 1 372.1265 101.8799)" fill="#FFFFFF" font-family="'ArialMT'" font-size="10">33</text>
    </g>
    ...
</g>
<rect id="myFrame" x="141.732" y="85.04" fill="none" stroke="#000000" stroke-width="0.5" stroke-miterlimit="10" width="283.464" height="141.732"/>
...

What we see here are three rectangle elements (top bar of the chart) with graphical x-coordinates and widths that lie within a rather arbitrary range. The same holds for three text elements (value labels of top bar) and their coordinates that are stored within a SVG transformation matrix in attribute ‘transform’. Their text entry is fixed to 33. All of these are grouped together (top bar with value labels) and then grouped again (all the bars). The last line shown corresponds to the rectangle that serves as the outer frame of the data area of the chart.

The following lines of code are enough to set coordinats, widths and numbers in the bar chart right when, for example, percentage values are stored within a dataframe of 5 rows (the groups) and 3 columns (the categories):

svg <- read_svg(file = "images/fig1.svg")
myValues <- data.frame(cat1=c(0.1,0.2,0.3,0.4,0.5),
                       cat2=c(0.35,0.25,0.35,0.25,0.35),
                       cat3=c(0.55,0.55,0.35,0.35,0.15))
svg <- stackedBar(svg = svg,frame_name = "myFrame",group_name = "myBars",
                  scale_real = c(0,100),values = myValues*100)
write_svg(svg = svg,file = "images/fig1_values.svg")

The result looks like this:

Fig. 2: SVG manipulated to reflect real values

Fig. 2: SVG manipulated to reflect real values

The magic happens at the function call for stackedBar. Here, one argument refers to the named rectangle ‘myFrame’ to define outer limits for the graphical coordinates and one argument to the named group of elements ‘myBars’ containing rectangles (bar segments) and texts (value labels) for the chart. Concerning the values one wants to show in the chart, the “real” scale is defined by a vector ranging from 0 to 100 and a dataframe with values is provided. svgtools can now calculate the corresponding graphical coordinates and widths and change the elements in ‘myBars’ accordingly.

This vignette explains how to set up SVG files so that svgtools can work with them and gives insight in the most common usage of package functions. For detailed information on all functions and arguments see ?stackedBar and others.

Reading, displaying and writing SVG files

Functions for file handling and display are rather straight-forward in svgtools. A typical workflow looks like that:

svg <- read_svg(file = "myFile.svg")
summary_svg(svg = svg)
display_svg(svg = svg)
# Code to manipulate the SVG
# ...
display_svg(svg = svg)
write_svg(svg = svg,file = "myFile_out.svg")

read_svg relies on read_xml from the xml2 package. So it has all the possibilities to read a file from the file system, a connection or even a raw vector. It defaults to encoding UTF-8, which may be changed with argument enc="latin-1", for example.

Function summary_svg is a convenience function that prints some useful information about the SVG content on the console (or whereever sink is set to). For the SVG in Fig. 1 the output would look like this:

[1] "************************"
[1] "** -- SVG SUMMARY: -- **"
[1] "************************"
[1] "-- NAMED GROUPS:"
[1] "myBars with 5 children"
[1] "-- AVAILABLE FRAMES:"
[1] "myFrame"
[1] "-- USED FONTS:"
[1] "'ArialMT'"
[1] "-- USED FONT SIZES:"
[1] "10"
[1] "-- USED COLORS:"
[1] "#C6C6C6" "#878787" "#3C3C3B" "none"    "#000000"

One can see that there is a named group (‘myBars’) in the SVG. It contains five child elements, that are the five bars of the chart, see further below. Also, there is one “available frame” (a named rectangle) called ‘myFrame’. This information helps with setting right the arguments in the function calls to manipulate the SVG. Further information on used fonts, font sizes and colors in the SVG only serves the purpose of validating the consistency of the design. One can invoke summary_svg directly by argument summary=TRUE of read_svg.

To display an SVG on the current viewport one may use display_svg. Standard viewport depends on operating system and IDE. For example, RStudio plots the image under the Viewer tab. By default, width and height of the bitmap (image) are derived from its content and the current DPI setting of the viewport. But one can set desired width and height with the correspondent function arguments.

Typically, display_svg is used before and after SVG manipulation to get visual proof of the changes. Therefore, argument display=TRUE of read_svg conveniently invokes the function.

Finally, write_svg uses write_xml from the xml2 package to write the (then manipulated) SVG to file system or an open connection. By default, hidden elements of the SVG are removed in the written file (not in the XML document in the R environment). To change this behavior set remove_hidden=FALSE. If one wants to remove all groupings in the written file (again, not in the XML document itself) it is possible to set flatten=TRUE. This may be beneficial in further layouting tasks on the resulting SVG image.

General principles of operation

svgtools relies heavily on naming objects of the SVG. One can always accomplish that with any text editor by inserting id-attributes in the XML element for the object. See the following:

<rect id="myFrame" x="141.732" y="85.04" width="283.464" height="141.732"/>

Naming an object in that way is also possible in almost any vector graphics program. (Check the manuals.) For example, in Adobe Illustrator using the Layers Panel to name objects ultimately leads to XML elements with id-tags when saving as SVG.

The following rules apply:

On the side of the values one wants to show in a chart, be mindful that svgtools does not calculate anything apart from the right coordinates and widths/heights of objects. This is relevant in situations like the following:

Adjustment of charts with lines and/or symbols needs a simple vector of numerical values. For bar charts, it is possible to adjust several bars at once. In that case, one needs to provide a dataframe or a matrix (with only numerical values). Then, rows always concern different bars, while columns define the sequence of bar segments to stack.

Horizontal and vertical alignment of charts works essentially the same. A corresponding argument is provided in all manipulating functions except changeText. Mind that alignment="horizontal" means adjusting x-coordinates for all chart types while alignment="vertical" always refers to adjustment of y-coordinates. This may be counter-intuitive when it comes to line charts, see below.

Adjusting bar charts

General bar charts

For a general bar chart one needs to prepare an SVG file that has named (XML attribute ‘id’) groups (XML element ‘g’) of bar segments (XML element ‘rect’) and, optionally, value labels (XML element ‘text’).

svg <- read_svg(file = "images/fig3.svg",summary = TRUE,display = TRUE)

Reading the SVG file with arguments summary=TRUE and display=TRUE conveniently prints information about it on the console and displays the SVG in the current viewport (see here). It might look like that: