XY Plotting > Overview of the XY Plot Application > Plot Specifications for Curves
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Plot Specifications for Curves
Each curve definition includes the following parameters that specify how the curve is plotted in an XY window where it is posted:
Start and End Points. The number of the first and last data point to be plotted. The default start point is data point 1 and the default end point is the last data point in the curve data.
Scale Factors. The X and Y data point values to be plotted are multiplied by the scale factors. The default scale factor is set to 1.
Frequency. This value specifies the interval between plotted data points. The default value of 1 is used when all data points between the start and end points are to be plotted. If a value (n) greater than 1 is specified, every nth data point is plotted. Thus, a frequency value greater than 1 produces filtering of the data.
Values of X. If the curve data type is Y Data (i.e., only Y data values are specified in the curve’s definition,) parameters for calculating the value of X for each plotted Y value are required. An X Initial Value and Delta Value are specified for this purpose. The initial value is associated with the Y value for the first plotted data point and the delta value is used to calculate the value of X for the remaining data points.
Curve Fit Method. Any of the following curve fit methods can be selected:
• Linear. The plot line for the curve consists of n-1 straight lines consecutively connecting the n plotted data points.
• Scatter. No plot line is generated. Only the symbols for the plotted data points are displayed.
• Least Squares. A parametric least squares numerical approximation of the best fit is performed, resulting in a curve consisting of one or more parametric cubic curves (PCs) that pass consecutively through (or near) the data points. The accuracy of the fit is determined by the user-specified number of PCs and the number of line segments per PC. Accuracy improves as the number of PCs and/or the number of line segments per PC are increased.
• Spline. The plot line for the curve consists of n-1 parametric cubic curves (PCs) consecutively connecting the n plotted data points. The resulting parametric cubic spline does not have to be monotonic. However, spline equations are often ill-conditioned and rapid changes in direction or large variations in spacing may produce “kinks” or “loops.” The curve fit accuracy is adjusted by changing the number of line segments per PC.
All plot parameters except the curve fit method are on the Modify a Curve’s Plot Specifications, 44. The curve fit method selection is on the Modify a Curve’s Fit Method and Display Attributes, 43. Both of these subordinate forms are accessed from the XY Plot form when Modify is the selected action and Curve is the selected object.