# HG-2000: Evaluate Curve Data

In this tutorial, you will learn how to evaluate curve data.

In this tutorial, you will:
• Reference curves
• Create curves using math expressions
• Create curves by using pre-defined functions
• Use the freeze option
The Define Curves panel can be accessed in one of the following ways:
• Click the Define Curves icon,
• From the menu bar select Curves > Define Curves

This panel allows you to edit existing curves and create new ones. The Define Curves panel also provides access to the program's curve calculator.

Referencing Curve Vectors in HyperGraph:

X and Y vector expressions can reference any curve vector in the session. A curve vector reference defines the x or y vector (values) by page, window, and curve number. An example of a curve vector reference is p2w3c4.x, where:
 p2 is page 2 w3 is window 3 c4 is curve 4 x is the vector
There are two common methods to specify curve vector references:
 Pick a curve in a plot window For the x = input field Shift and pick the curve to get the curve x vector reference. Shift + Ctrl and pick the curve to get the curve y vector reference. for y = input field Shift and pick the curve to get the curve y vector reference. Shift + Ctrl and pick the curve to get the curve x vector reference. Select a curve vector reference from the Curves… dialog, which is accessed from the Define Curves panel.
Freeze Option
The Freeze option is available when a vector’s source is Math. This option allows you to break the association of the dependent curve(s) from their parent curve, which allows modifications to the parent curve without updating the dependent curve(s).
Curve Dependencies browser
To access the browser, right-click a curve and select View Dependencies. The selected curve displays from the context menu.

## Open Session File democ2.mvw

1. From the File menu, click Open > Session.
2. Select the democ2.mvw file, located in the plotting folder, and click Open.

## Practice Referencing Curve Vectors by Selecting Curves from the Plot Window

1. Make window 3 (lower left window) on page 2 active by clicking on it.
2. Expand window 3 by clicking the Expand Window button, .
3. Go to the Define Curves panel, .
The Curve list displays the names of the four curves in window 3.
4. Click Add under the curve list.
5. Verify the x = radio button is selected.
6. Select Math under Source of the curve to be created.
This defines the x vector of the new curve to be a math function. In this case, it will simply be set equal to the x vector of an existing curve.
7. Hold the Shift key and pick the dark red curve (Req/5 Curve 5).
The x = curve reference is p2w3c2.x. This defines the page 2, window 3, curve 2, x vector.
8. Clear the entry in the x = field.
9. With the x = field still active, hold the Shift + Ctrl keys and pick the same curve.
The x = curve reference is p2w3c2.y. This defines the page 2, window 3, curve 2, y vector.
10. From the curve list, select Curve1 and click Cut.

## Create a New Curve with a Math Expression for the Y Vector

1. Click Add to create a new curve named Curve2.
2. Verify the x = radio button is selected.
3. Select Math for the Source:.
4. Click Curves… to open its dialog, and make the following selections:
1. p2: Plot 2
2. w3: Plot
3. c2: Req/5 Curve 5
5. Click Select to complete the selection
The result is the same as when the curve was picked from the graphics area; the x curve reference is p2w3c2.x.
6. Click the y = radio button.
7. Hold the Shift key and pick any curve in window 3.
Note: In the y = field is the reference for the y vector of the picked curve.
8. Append the character + to the string in the y = field.
9. Hold the Shift key and select any other curve in window 3.
Note: The y = field now has two y vectors separated by the + sign.
10. Click Apply to create the new curve.
The new curve appears in window 3 and is the sum of the two y vectors.