Display a legend for all charts displaying multiple lines

[alsargent]

Template: All, but especially the MySQL/Nginx, Docker, and Postgres/Apache templates.

Describe the problem
Legends aren't being displayed for charts with multiple lines. As a result, it's not clear what you're looking at.

To Reproduce

For example, look at the Nginx / MySQL template on https://github.com/influxdata/community-templates/blob/master/nginx_mysql/img/Dashboard.png:

There's a chart titled reading/writing/waiting. We see two lines plotted. But it's unclear which line is for reading, which is for writing, and which is for waiting.

Similarly, check out the Apache/Posgres template at https://github.com/influxdata/community-templates/tree/master/apache_postgresql:

There are several lines in the Connections chart. What do they all mean?

In the Docker template (https://github.com/influxdata/community-templates/tree/master/docker) we have a chart titled Number of Docker containers, which multiple lines. I'm guessing each line corresponds to a host, but I can't tell which line is for which host:

Expected behavior
When a chart has multiple lines, display a legend that indicates what each line corresponds to.

[alsargent]

@mhall119 I believe this auto-assigned to you as soon as I submitted this. Feel free to assign to someone else if needed.

[mhall119]

@alsargent the charts themselves have a legend, are you asking that we include that in the screen shots? Or that we provide the legend in the README for each template?

[mhall119]

Closing this based on past discussions, InfluxDB 2 doesn't currently show a legend that we can grab in the screenshot