Can I Create a Graph (Or Perform Further Calculation) On the Summarized Results?

You cannot perform further calculation on the summarized results using the calculation/graph features. For example, you cannot calculate percentage of two summarized values in certain fields.

However, if you want to show the total sales amount and the achievement rate on the same screen, you can place fields for the sales amount, target amount, and achievement rate respectively on the form, and then calculate "sum" for the sales amount and the achievement rate and calculate "average" for the target amount.
In this case, it is recommended to select "Table" as a chart type, instead of "Bar chart". This is because it is going to be difficult to view the calculated achievement rate on the graph when the number of digits is hugely different between the calculated values in the achievement rate and the sales amount fields.
Examples of configurations are as follows.

First, place the following fields on the form of the app.

Example of field settings
Field Name Field Code Field Type
Date Date Date
Contact Name Contact_Name User selection
Monthly Target Amount Monthly_Target_Amount Number
Sales Amount Sales_Amount Number
Achievement Rate Achievement_Rate Calculation
- Formula: Sales_Amount / Monthly_Target_Amount * 100
- Unit of measure: % (Suffix)

Configure a graph for calculation. You can specify any name for the graph.
Refer to "Creating Graphs"

  
Example of graph settings
Chart Type Table
Group by Level 1: "Date" "by the month"
Level 2: "Contact Name"
Function "Average" "Monthly Target Amount"
"Sum" "Sales Amount"
"Sum" "Achievement Rate"
Filter All records
Sort By "Level 1" "Ascending"

Add the following records.

Example of added records
Date Contact Name Monthly Target Amount Sales Amount Achievement Rate
2021-07-01 A 300000 20000 6.67%
2021-07-08 A 300000 15000 5%
2021-07-12 A 300000 30000 10%
2021-07-05 B 250000 10000 4%
2021-07-15 B 250000 50000 20%

Check the summarized result.
Refer to "Summarizing"

Example of summarized records
Date (by the month) Contact Name Average (Monthly Target Amount) Sum (Sales Amount) Sum (Achievement Rate)
2021-07-01 B 250000 60000 24%
2021-07-01 A 300000 65000 21.67%

In this way, you can show the sales amount and achievement rate on the same screen.