Can't set xsl:variable using substring-after from attribute

I have very limited experience with xslt so I can't seem to find the problem here.

In the XML I have multiple occurrences of distinct graphic elements like so:

<graphic xlink:href="someVariableText-gf01.tif"/>
<graphic xlink:href="someVariableText-gf02.tif"/>
<graphic xlink:href="someVariableText-gf03.tif"/>

I need to change the xlink:href preserving the unique numbers after -gf.

My code below:

<xsl:template match="graphic/@xlink:href">
    <xsl:variable name="aftergf" select="substring-after(., 'gf')"/>

    <xsl:attribute name="xlink:href">
    <xsl:value-of select="concat('MyNewText-gf', $aftergf)"/>
    </xsl:attribute>
</xsl:template>

Gives me:

<graphic xlink:href="MyNewText-gf"/>
<graphic xlink:href="MyNewText-gf"/>
<graphic xlink:href="MyNewText-gf"/>

What am I doing wrong?