Hello James

These are most likely dates within a month that contain values, so they are being displayed on the X axis.

This behavior is coming from the chart engine, not wpDataTables, and to change it, you'll probably need to use some wpDataCharts callbacks. 

• Every chart exposes several options that customize its look and feel. Charts usually support custom options appropriate to that visualization. wpDataChart callbacks allow adding options that are available in Google Charts API, Highcharts API, and Chart.js API. 

• All necessary resources are available in charts engines API (depends on which one you use). Every engine has a different approach to chart settings. In wpDataChart callbacks, you have to adopt those settings to the wpDataChart object (you can take a look at examples for each engine in our documentation, and also in the Support help center). A huge number of examples for any area of programming can be found on stackoverflow.com (typing your problem in google and at the end add "site: stackoverflow.com" and Google will search only that website). Also, a lot of examples of charts, chart settings, and customization can be found on jsffidle.net (typing in google for example "line chart highcharts jsffidle")
• Please note that using hooks or wpDataTable and wpDataChart callbacks requires a certain level of programming skills and included support refers only to advice.