Notes on Sigmaplot for plotting and curve fitting


Sigmaplot is a computer package for data plotting and data analysis. You may use this program in the Bioeng lab to make plots for your writeups, as well as the curve fitting required for the viscoelasticity and other labs.

Making a graph:

To plot data, first you must enter the values (X and Y data as columns) into a blank data sheet. Notice that Sigmaplot stores data in a "notebook" which contains both the data spread sheet and any graphs that you create. You can type the numbers in by hand into the cells, or copy and paste from other programs such as Excel, or import the data from a text file. For example the output of Labview may be imported directly using the "import/Plain Text" option. Once you have your X-Y data in a data file, use the graph:create graph menu and follow the steps. Enter the data format as "XY pair" then select the columns from your data sheet to be used as the X and Y values. Once the plot is made, you can edit most objects on the plot by double clicking on them, and if you double click on the graph itself, you can edit symbols, axes, etc by clicking in the appropriate box in the graph properties window. Remember your plot will be saved as part of the notebook, so you may have to open that particular window to see it next time.

Curve fitting:

Sigmaplot has extensive curve fitting functions which will come in very handy for the muscle II & viscoelasticity labs. To fit a curve, have your X-Y data in a Sigmaplot data sheet as described above. Click on the statistics:regression wizard... menu; select the type of curve fit, click on the "next" button and select your X,Y columns, then "next" again to get the constants in the fit. You should always verify a computer curve fit by plugging in a couple points by hand and check for accurate fitting.

For example, for a hyperbolic function such as Hill’s equation, the best fit will be with the "Rational" type function, in this case "3 Parameter I". Notice you will have to do some computations to make the constants match those in Hill’s equation. For a creep function, use the "Exponential Rise to Max" fit with "single, 3 Parameter" to fit your function. You will have to delete the "baseline" data up to the time of the step for the fitting procedure to work properly.

Link to the Sigmaplot web page