Update scatter chart default to hide lines

[etimberg]

Resolves #4372

Note: This is a potentially breaking change if we think that scatter defaults should show lines. In which case, I will update the two sample files

[simonbrunel]

Seems like a breaking change if we change defaults?. About the scatter chart definition, I really don't know what it should look like.

[etimberg]

Right. In that case, I can just change the samples and we can change the defaults for v3.

[simonbrunel]

When searching scatter chart over the web, all results look like multiple points/symbols without lines (so the expected behavior described in #4372). So may be better to change the default in 2.7 and justify the breaking change as a bug fix?

@brylie @kumarharsh thought?

[kumarharsh]

Personally, I've never seen a scatter chart with lines - yes, it is based off the line chart, but i think that's where the similarities end.

There is another change which needs to be done, which was not clear to me at the start - both the axes need to have numeric scales by default in the scatter chart, which the line chart doesn't have by default.

[simonbrunel]

I vote for the breaking change then, and change defaults :) @etimberg what do you think?

[etimberg]

I'm fine with changing the defaults. Will update this PR accordingly

[openjck]

If anyone else comes across this issue looking for how to restore lines for scatter charts in chart.js, just add showLines: true to the configuration object.