It takes courage to post a detailed question like this in a public forum. You have the data, you know the story you want to tell, individual progress across three measurement points, and yet the tool is fighting you at every turn. Excel is treating your values as labels, your lines as columns, and your graph as a single point. You are not doing anything wrong. You are running into a fundamental mismatch between what traditional spreadsheets expect and what you actually need to see.
This frustration is not your fault. It is the natural outcome of a tool designed for accountants in the 1980s trying to handle modern, people-centered data. You have three groups, twenty people per group, and three time points. That is a simple longitudinal dataset. In Excel, however, the moment you import data from SPSS and try to visualize it, you are forced to fight axis formatting, selection logic, and a chart engine that assumes your data is a flat table of categories. The result is a graph that shows nothing useful and a user who feels incompetent. You are not incompetent. The tool is asking you to translate your human question, how did each person change over time?, into a machine language that was never built for that question.
Here is what we think: the problem is not your data, and it is not you. The problem is that Excel expects you to think like a spreadsheet, not like a researcher or a manager who cares about people. You need a tool that understands your intent: twenty lines, one per person, with time on the horizontal axis and the measurement value on the vertical. That is a straightforward ask. A modern, AI-native spreadsheet would let you describe that in plain language and generate the graph on the first try. It would not require you to know that your data needs to be in "wide format" or that Excel's default chart type assumes your first column is a category axis. It would look at your three columns of measurements, recognize the longitudinal structure, and offer you a line graph with individual trajectories grouped by intervention condition.
What this means for you in practical terms is that your time is better spent interpreting the graph than wrestling with the software. The chart you want exists. It is a simple matter of finding a tool that treats your question as the starting point, not as a puzzle to solve. Explore a platform that lets you import your SPSS data and immediately visualize individual progress without reconfiguring axis labels. The graph you need is one click away, once you stop asking Excel to do what it was never designed to do.