Often times when building a Dual Axis Map we end up with Null marks being shown on the other axis. When we filter out these Nulls on one axis, it also filters out the associated marks on the other axis -- not what we are looking for. While we can't use filters to eliminated the dots, we can hide them by using a "Hide" option.
Here is a dual axis ZIP Code & Locations map that demonstrates the issue:
Note all the extra dots in the filled ZIPs. We can get rid of most of these by changed the lat/longs from Dimensions to Measures. Like this:
Then to get rid of this single dot we use Hide (steps described below). Here's what the finished viz looks like:
In the attached workbook you will see that the ZIPs and the Locations share the same lats & long fields and that we use the Type field to separate them so we can put each on a different axis. Here are the steps needed to create this viz:
- Create the two separate geographies using these formulas
- ZIPs Only: IF [Type]='ZIP THEN [Zip Codes]
- Locations Only: IF [Type]='Locations THEN [Zip Codes]
- Create a dual axis map as shown in the workbook tab "before hide"
- To get rid of the dots, first move ZIPs Only to the Size shelf
- Right-Click the Null value in the Size legend and click "Hide"
- Now move the ZIPs Only from the Size shelf back to the Detail shelf. This pill will continue to hide the Nulls.
- Do the same thing for the Locations Only pill on the other axis
The only downside to this technique is that someone trying to edit your workbook in the future may have difficulty figuring out how you hide the Nulls, so it may be a good idea to document this in the caption.
[Feel free to edit this TabWiki.]