maxPadding property

Padding of the max value relative to the length of the axis. A padding of 0.05 will make a 100px axis 5px longer. This is useful when you don't want the highest data value to appear on the edge of the plot area. When the axis' max option is set or a max extreme is set using axis.setExtremes(), the maxPadding will be ignored.

Also the softThreshold option takes precedence over maxPadding, so if the data is tangent to the threshold, maxPadding may not apply unless softThreshold is set to false.

API Docs: https://api.highcharts.com/highcharts/chart.parallelAxes.maxPadding