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To create a contour plot, choose Analyze:Contour Plot ( Z Y X ). If you have already selected three or more numeric variables, a contour plot for each unique triplet of variables appears. If you have not selected any variables, a variables dialog appears. In the dialog, select at least one Z, Y, X variable.

You can select one or more Group variables if you have grouped data. This creates contour plots for each group.

You can select a Label variable for labeling observations in the plots.

Observations with missing values for Z, Y, X variables are not used. If several observations have the same (x, y) values, the contour plot is calculated using the mean Z value at (x, y).

Clicking on the Method button in the variable dialog enables you to set several options for the contour plot computation:


Figure 1.5: Contour plot methods

Grid Size:Horizontal Size
specifies the horizontal resolution of the rectangular grid over which the contour function is evaluated.

Grid Size:Vertical Size
specifies the vertical resolution of the rectangular grid over which the contour function is evaluated.

Fit:Linear Interpolation
linearly interpolates the contour function across rectangular grid cells.

Fit:Thin-Plate Smoothing Spline
fits the contour function over rectangular grid using thin-plate smoothing spline fitting. The process may be much slower than linear interpolation. It usually produces very smooth contours.

Number of Contour Levels
specifies the number of contour levels to be drawn in a contour plot. The contour levels are spaced evenly within the range of the Z variable.

You can modify other aspects of a contour plot by using the pop-up menu.

specifies tick labels on either axis.
toggles the display of axes.

toggles the display of observations. When this menu is toggled off, observations are displayed only if selected.

Reference Lines
toggles the display of reference lines.

Contour Lines
toggles the display of contour lines.

toggles the display of contour level labels.

Fill Areas
toggles the display of filled contour areas. When this menu item is toggled on, an area between two adjacent contour levels is filled in the color of the lower level.

Color Blending
applies color blending to all contour levels. The color blend in the tool window is used.

Marker Sizes
sets the size of markers used to display observations.

You can select and brush observations in the contour plot even when they are not visible. If you click on a line at the location of an observation, you select that observation. If you click on a contour line between two observations, you select the contour line.

You can use the hand tool to add and move contour lines and thus change contour level settings. To add a line, click at the location where you want the line to be. To move a line, drag on the line, then release the mouse at the new location.

Finally, you can set colors, patterns, and widths of contour lines the same way you set these attributes for curves.

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