Sorting legend does not work correctly in some cases

[qn895]

Describe the issue

Sorting legend does not work correctly in some cases when there are Area series preceding Line series (or possibly when there are simply multiple series?)

To Reproduce

Case 1:

1. Go to https://codesandbox.io/s/xenodochial-ptolemy-w5ftu4?file=/src/App.tsx
2. Toggle between "Enable legendSort" & "Disable legendSort" button
3. Note that the legend is being sorted correctly/as expected. When sortLegendEnabled === true, Expected bounds should be after Throughput.

Case 2:

1. Go to https://codesandbox.io/s/recursing-euclid-rjr3x0?file=/src/App.tsx
2. Note that the legend is NOT being sorted. Even though it uses the same legendSort function as Case 1, Expected bounds still show up before Throughput.

Checklist

• [x] The proper chart type label has been added (e.g. :xy, :partition)
• [x] The proper feature labels have been added (e.g. :interactions, :axis)
• [x] Every related Kibana issue is listed under Kibana Cross Issues list and the kibana cross issue tag is applied