In a bar chart (clustered), the origin of the vertical axis is the bottom of the axis.
Therefore, if you perform a sort by Level 2 or Level 3, the items are displayed in ascending or descending order from the bottom of the vertical axis of the chart.

This article explains how to sort items in a bar chart (clustered) using examples.

In your app, place the fields used to create a chart.

• Select the Form tab on the App settings screen, then place Text, Number, and Check box fields on the app form.
• In the Check box settings screen, add the following items under Options.
  sample1
  sample2
  sample3
  sample4

Save the form and update the app.

Add the following records.

• Text field: Type "test"
  Number field: Type "1"
  Check box field: Select sample1
  
  

• Text field: Type "test"
  Number field: Type "3"
  Check box field: Select sample2
  
  

• Text field: Type "test"
  Number field: Type "1"
  Check box field: Select sample3
  
  

• Text field: Type "test"
  Number field: Type "2"
  Check box field: Select sample4

On the View screen, click the Create graph icon to add a chart. You can add a chart from the Graphs tab on the App settings screen.

• Chart type: Bar chart/Clustered
• Group by: Level 1 "Text", Level 2 "Check box"
• Function: Sum/Number
• Sort by: Level 2/Ascending

In the Create graph screen, click Apply to create a graph as shown below.

• The bottom of the vertical axis is set to the origin of the sort. Thus when you sort graph items "by Level 2 in Ascending order", items are displayed in the order from sample1 to sample4 (from the bottom to the top).
• The items in the legend is displayed from sample 1 to sample4 (from the left to the right) as you sort the graph items "by Level 2 in Ascending order".