XY comparison

Revision as of 20:02, 6 August 2007 by Sswamy (talk | contribs)

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Normally, a graph plots the values of a variable V along the vertical (Y) axis against one of its indexes along the horizontal (X) axis. XY comparison means plotting the values of a variable V along the vertical (Y) axis against the values of another variable along the horizontal (X) axis.

Use XY coordinate [comparison] to create a scatter plot

For example, if you graph the sample view of an uncertain variable V, it normally plots the values of V on the vertical axis and the Run index (1 to Samplesize) along the horizontal axis. In a scatter plot, you can plot the values of V vertically (Y) against another uncertain variable U plotted horizontally (X). Each of the sample size point is a sample of values from U and V. We term Run the common index, being common to both U and V.

[Text in square brackets refers to possible future change in names and text of menus and dialogs.]

Suppose V and U are uncertain and dependent variables: 

Variable V := Normal(5, 2)
Variable U := V + Uniform(-1, 3)


Here is how to use XY Coordinate sources to create a scatter plot of V against U:

  1. Display the graph for uncertain quantity V. [img]
  2. Select the Sample view. [img]

Sample_view_button.png

  1. Make sure you are in Edit mode, so that the XY button is displayed in top right of the Graph window. [img]
  2. Click the XY button to open the XY Coordinate sources [XW Comparison setup] dialog. [img]
  3. Select checkbox Use another variable [Compare against these variables with a common index:]
  4. Click Add… to open the Object finder
  5. In the Object finder, select variable U, and click OK.
  6. U is now in the Use another variable list in the XY Coordinate sources. [Note that the common index is set to Run.] Click OK. [img]
  7. The graph now shows the scatter plot of V on vertical (Y) axis and U on the horizontal (X) axis. Each point is a random sample over index Run (from 1 to 100). [img]

Common index

Each point on the graph represents a value of the common index. The common index should be common to the current variable V and other variables or expressions to which you wish to compare V. If there is more than one such index, it shows the common index as a menu so that you select the one you want. By default, it selects what it considers the most plausible, usually the index with most elements.

Use coordinate index [Use comparison index]

Check this box if you want to compare one slice of the current variable against another slice, with each slice selected from the coordinate [comparison] index. The coordinate index is an index of the variable. After clicking OK to return to graph window, you will be able to select slices from the common index from the Horizontal and Vertical menus to display along those axes.

Horizontal axis

This menu lets you select which variable to plot on the horizontal (X) axis. It appears in the result window when there is more than one alternative variable to display, for example, if there is more than one variable or expression in the Compare with list – or if you have selected a Comparison index, it lets you select which slice you want to plot.

Vertical axis

This menu lets you select which variable to plot on the vertical axis. It appears on the result window when you have selected a comparison index from the variable. It lists the slice values from the comparison index, from which you can choose the one to plot along the vertical (Y) axis.

XY coordinate sources [Comparison set up] dialog

Add…

You can compare the variable not only with other variables, but also with simple expressions calling a function or operator. 1. Click Add.. to open the Object finder. 2. Press on the Library menu, and select the library (or module) that contains the function or operator you want. 3. Select the function or operator, and fill in its parameters (or operands) 4. Click OK to add expression to Other variables and expressions.

Remove

Select one or more variables or expressions, and click Remove to remove them from the list.

Use coordinate index

You can compare (plot against one another) slices of the current variable over the coordinate [comparison] index.

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