Updated Hjulstrom curve to SVG format.

This update also improves readability for those suffering from colour blindness.
Updated svg-diagram-styles to only apply styling to text inside a group called labels.

This allows separate styling of other text (e.g., axis labels). Will require updating several diagrams.
Updated svg-diagram-style to apply styling to all text descendants of a #axis-labels group.
Added fill property to svg-diagram-style.
Removed fill property from svg-diagram-style

I quickly realised this is a bid idea. Stupid me.
Updated old SVG diagrams to support new svg-diagram-style.