Moodle document display converter misrepresentation of scatter graphs

For an assignment requiring a scatter graph, a chart such as the following is a sign that the student has incorrectly chosen a line chart with points displayed instead of a line. 

The data that was used to generate the chart was as the following table.


In a scatter graph the x-axis should be labeled with equally spaced increasing number values, not the x-axis values. A scatter graph also usually has a grid of vertical and horizontal grid lines. The line chart may not. A chart such as the one above would be an indication that the wrong graph type was chosen.


This is the scatter graph for the same data. The x-axis is not labeled with data values from the first column and there are vertical grid lines.


This is a graph as displayed in the Moodle assignment grader. The resolution is as displayed in the grader. This would appear to be a line chart type, not a scatter graph. 

Upon downloading the assignment, however, the chart is revealed to actually be a true scatter graph chart. Not a line chart. The second chart seen above is the very same chart as the one displayed in the grader above. Somewhere in the conversion process the converter has altered the x-axis and dropped the vertical grid lines. 

This leads to the instructor flagging the assignment for not having the correct chart type and instructions to use a scatter graph chart type. Which is what the student actually did, thus leaving the student very confused. 

While the downloaded spreadsheet is downloaded by Moodle as an Excel format spreadsheet, the original document was a Google Sheets file generated using copy on link to generate the student's document. The converter being used by Moodle converts the gsheet file format to an Excel format and then, for display purposes, appears to convert the Excel file to an annotatable PDF file. Somewhere in this chain of conversions the original scatter graph takes on the appearance of a line chart.

This means that marking spreadsheet assignments that contain scatter graph charts requires downloading each and every submission to accurately determine the chart type in use. 

This issue is known to occur with spreadsheets being imported from Google Sheets in Moodle. Exploration beyond Google Sheets has not been done. 

Post-script 12 February 2025

The Moodle grader will display the x-axis reversed from what was submitted.

Above is the scatter graph as submitted by a student.


The same graph as displayed in the Moodle grader. The x-axis has been reversed.


Another graph as submitted by a student.


The same chart as seen in the Moodle grader. Note that in this case the grader not only reversed the x-axis, but it also shifted to even spacing of the data points, effectively a line graph style for the x-axis.

The conditions, software, and operating system variables that may or may not contribute to this are not known. On a resubmission of an assignment which had a reversed x-axis for the first submission, a resubmission did not have a reversed x-axis. This inconsistency in reproducibility means that tracking this bug down is probably nigh on impossible.  

Second post-script

To give credit where credit is due, a student, Maydelle F., solved the above x-axis reversal part of the puzzle. For her first submission she submitted the following.


The Moodle grader displayed this as with a reversed x-axis as seen below.


The x-axis has been reversed. Maydelle then submitted the following. 


This then appeared as follows in the Moodle grader. 


The axis is no longer reversed on the graph. Did you catch the change Maydelle made? This author did not see the difference at a first look. If you are not seeing the change, look more closely at the table for the original submission versus the subsequent submission. 

This does not resolve the issue that the converter is converting from a true scatter graph to an equally spaced line style chart. 


MZ submitted the above spreadsheet. Note that the drop heights are not equally spaced.


In the Moodle grader, however, the data points are evenly spaced along the x-axis dimension. The axis was not reversed due to the table order.

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