$m = replaced with measurement name
$measurement = replaced with measurement name
$1 - $9 = replaced with part of measurement name (if you seperate your measurement name with dots)
$col = replaced with column name
$tag_hostname = replaced with the value of the hostname tag
You can also use [[tag_hostname]] pattern replacement syntax

When stacking is enabled it important that points align
If there are missing points for one series it can cause gaps or missing bars
You must use fill(0), and select a group by time low limit
Use the group by time option below your queries and specify for example >10s if your metrics are written every 10 seconds
This will insert zeros for series that are missing measurements and will make stacking work properly

Group by time is important, otherwise the query could return many thousands of datapoints that will slow down Grafana
Leave the group by time field empty for each query and it will be calculated based on time range and pixel width of the graph
If you use fill(0) or fill(null) set a low limit for the auto group by time interval
The low limit can only be set in the group by time option below your queries
You set a low limit by adding a greater sign before the interval
Example: >60s if you write metrics to InfluxDB every 60 seconds