[Feature] Legend selectedMode to keep at least one active

[dileepyelleti]

What problem does this feature solve?

When all series are disabled from legend chart looks empty and even y-axis disappears. https://echarts.apache.org/examples/en/editor.html?c=line-stack - disable all series from legend

What does the proposed API look like?

Add one more option to selectedMode(inside legend) which disables unselecting last active series. legend: { selectedMode: 'atLeastOne' }