Create SVG Polyline Graph Using XML and XSL

Question

I am trying to create a SVG polyline graph from values within my XML doc. So far I have produced the chart but I am unable to correctly retrieve the values from the XML doc. Below is the current state of the graph:

The graph is intended to display average monthly weather temperatures so should therefore be lower at the edges and a peak in the centre.

Here is my XML code:

<xml>
<graph2>
    <averageHighTemperatures>
        <January>8.3</January>
        <February>8.5</February>
        <March>11.1</March>
        <April>13.5</April>
        <May>17.1</May>
        <June>20.0</June>
        <July>22.6</July>
        <August>22.5</August>
        <September>19.3</September>
        <October>15.3</October>
        <November>11.2</November>
        <December>9.1</December>
    </averageHighTemperatures>
</graph2>
</xml>

Here is my XSL code:

<svg xmlns="http://www.w3.org/2000/svg" 
         viewBox="0 0 500 100" class="chart">

        <xsl:variable name="max">
            <xsl:for-each select="xml/graph2/averageHighTemperatures/*">
                <xsl:sort select="." data-type="number" order="descending"/><xsl:if test="position()=1">
                    <xsl:value-of select="."/>
                </xsl:if>
            </xsl:for-each>
        </xsl:variable>

        <xsl:for-each select="xml/graph2/averageHighTemperatures">
            <polyline
                fill="none"
                stroke="#0074d9"
                stroke-width="3"
                points="0,{January div $max}
                        20,{February div $max}
                        40,{March div $max}
                        60,{April div $max}
                        80,{May div $max}
                        100,{June div $max}
                        120,{July div $max}
                        140,{August div $max}
                        160,{September div $max}
                        180,{October div $max}
                        200,{November div $max}
                        220,{December div $max}"/>

        </xsl:for-each>
    </svg>

I assume that the XSL for each loop at the bottom is the source of the issue but I am unsure how to use the variable correctly when plotting the points.

Thanks in advance

Answer 1

In an SVG chart, 0,0 is at the top left. Smaller Y values will plot at the top, and larger Y values will move towards the bottom.

So, for your temperature line, the highest temperature should have the lowest number for the Y axis. You could divide the temperature value by the maximum and then multiple by 100, and then subtract that from the height of the chart. Since you have a stroke-width, the line for the highest numbers might get cut off, so you might want to account for that in the calculation of the values.

This is how I would generate your linechart:

<xsl:variable name="stroke-width" select="3"/>
<xsl:variable name="max" select="max(xml/graph2/averageHighTemperatures/*)"/>
<xsl:variable name="box-height" select="100"/>  
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 600 {$box-height}" class="chart">
  <polyline
    fill="none"
    stroke="#0074d9"
    stroke-width="{$stroke-width}" >
    <xsl:attribute name="points">
      <xsl:for-each select="xml/graph2/averageHighTemperatures/*">
        <xsl:value-of select="20 * (position() - 1)"/>
        <xsl:text>,</xsl:text>
        <xsl:value-of select="$box-height - ((. div ($max + $stroke-width) ) * 100)"/>
        <xsl:text> </xsl:text>
      </xsl:for-each>
    </xsl:attribute>
  </polyline>
</svg>

Which renders as:

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 600 100" class="chart">
    <polyline fill="none"
              stroke="#0074d9"
              stroke-width="3"
              points="0,67.578125 20,66.796875 40,56.64062500000001 60,47.265625 80,33.203125 100,21.875 120,11.71875 140,12.109375 160,24.609375 180,40.234375 200,56.25000000000001 220,64.453125 "/>
</svg>