How to make a scatter plot
- Copy the X column and Y column (and a group column if needed) with their headers.
- Click the column header (A) in the data table and paste - the first line becomes the column names. You can also open a CSV or Excel file.
- Choose the X column and one or more Y columns, then the trendline function (linear, quadratic, exponential, logarithmic).
- Press Draw and check the equation, R² and automatic interpretation. You can edit the trendline coefficients in the result window.
What is a scatter plot?
A scatter plot draws each pair of values as a point to show how one variable changes as the other grows. Points rising to the right indicate a positive correlation, falling to the right a negative correlation, and a shapeless cloud little or no correlation.
In quality work it is often the first step in finding how process conditions (temperature, pressure, time) affect quality characteristics (yield, thickness, strength). Choose a group column to color points and fit trendlines separately for each machine or line.
Trendline types and R²
Linear (y = a·x + b) and quadratic (y = a·x² + b·x + c) trendlines are fitted by least squares, exponential (y = a·e^(b·x)) by non-linear least squares, and logarithmic (y = a·ln(x) + b) using only points with x greater than 0.
R² (coefficient of determination) shows, from 0 to 1, how much of the variation in Y the trendline explains. The closer to 1, the more tightly the points follow the line.
How to read a scatter plot
- The automatic interpretation calls a correlation strong when the linear R² is 0.7 or higher and weak when it is 0.3 or higher.
- Correlation is not causation - check whether another factor affects both variables.
- Points far from the trendline may be measurement errors or special conditions worth investigating.
- If the points curve, compare the R² of quadratic, exponential and logarithmic trendlines with the linear one.
Frequently asked questions
Can I plot several Y columns on one chart?
Yes. Select several Y columns to overlay them in different colors, or choose 'Plot each in a separate window' to draw them separately.
Can I get the correlation coefficient (r)?
The R² of the linear trendline equals the square of the correlation coefficient r. Pearson/Spearman correlation with p-values is available in the statistics menu of the Qstat desktop program.
What happens with blank or text cells?
Rows where X or Y is not a number are simply left out.
Is my data sent to a server?
No. All calculations and charts are produced in your browser and the data is never transmitted or stored.
Other chart tools
Need process capability, control charts or DOE?
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