Add axis values for the droplines/crosshair

[IPWright83]

@curran suggested adding in values on the axis when using droplinesa /crosshair which is a great idea.

Look at https://developer.mozilla.org/en-US/docs/Web/CSS/paint-order as a way to achieve a nice contrast

[IPWright83]

This is relatively easy to add into the layers, if we know the value to be added. At the moment the crosshair purely works from the mouse position.

To make this work, it'll need to use those co-ordinates through a d3.scale inversion. At the moment though it's not clear which Scale the crosshair would use. We need to either:

• Pass 1-2 scales into the Crosshair component
• Append horizontal/vertical meta-data onto scales so we can add new selectors to get this information for the Crosshair.

Note that also d3.scaleBand doesn't have an invert function. Could look at using https://stackoverflow.com/a/38746923/21061