I created a column chart and I want to add a line on the y-axis as the upper limit that increases by 300 every 6 hours (pink line in image attached). How would I do this?
-->Is there an option display line I can add in dashboard under my chart code?
I'm not entirely clear on whether you need the line to change value or be drawn straight across like in the graphic you attached.
For a static value, following the rest of your search that generates the above chart, do an eval to set a new field value to the limit your wanting to graph.
<YOUR SEARCH> | eval limit=300
Then under the format, Chart Overlay, click in the Overlay box and select the Limit field. This will draw that value across the chart.
If you need that line's value to change depending on other factors, including time, you can just add the appropriate calculation to limit= <calculation>.