In a bar chart (clustered), the origin of the vertical axis is set to the bottom of the axis.
Therefore, if you perform a sort by Level 2 or Level 3, ascending/descending order of the chart items will be determined based on the origin.
A detailed example is as follows.

Configure the App Settings to display the 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 the "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

Click the Create Graph icon on the "View" screen and configure the following settings. You can also create a graph 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".