There are many ways you can improve readability. Some of them:

1. color the axis labels:

labels: {style: {color: 'color matching series color'}},

2. same way you can color axis title
3. you can move legend and increase the size of legend symbols to make it more visually illustrative
4. you can add annotation or render labels with SVG Renderer: https://www.highcharts.com/docs/advanced-chart-features/renderer

Demo of first 2 steps: https://jsfiddle.net/BlackLabel/v89r6jh4/