Horizontal grid lines: adding these more frequently

[CGiattino]

Core problem

Grapher frequently shows data points that rise above the highest horizontal grid line (see example below), sometimes high above.

This makes it difficult to visually tell the value for those data points, especially for static charts (it's slightly less of an issue with interactive charts because the user can hover the value, but still not ideal).

This is the case for multiple chart types, really any that have horizontal grid lines.

Proposed solution

When a data point rises to 10% or more above a given horizontal grid line, add the next horizontal grid line above it.