Working with equations : Graph function

Graph function

The Sketch Graph window is called from the equation dialogue when the modeller wishes to sketch, by hand, the relationship between two variables. The relationship is, mathematically, a function: that is, there is only one Y value possible for any given X value.

The graph is sketched using the mouse. The graph consists of a number of straight-line segments. The x-coordinates of the two ends of each segment is fixed, but the y-coordinate of each end can be dragged up or down with the mouse. The y-coordinate at the end of one segment is constrained to be the same as the y-coordinate at the start of the next, making a continuous, if angular, curve. The modeller can vary the number of line segments used to make up the curve: if this number is quite high, then a close approximation to a smooth curve can be obtained.

The function is actually evaluated using linear interpolation. On any one occasion, there will be a single value for the x (independent) variable. The function calculates which line segment this occurs in, and how far along the line segment it lies. Simple algebra is then used to calculate the corresponding value for the resulting dependent (y) variable. Alternatively, you can choose to have the exact y value for the closest defined x value to the one given; see "options" below.

The window itself does not show the names of the variables involved. The independent (X-axis) variable is not yet specified at this stage, since that only happens when the user returns to the Equation window. The x value can be any Simile expression. There does not need to be a specific variable associated with y-axis, since the result of evaluating the graph function is just like any other function result - a value to be incorporated into the rest of a mathematical expression: graph(weight) plays the same role in an equation as log(weight), for example.

Click the OK button to return to the equation dialogue window. New text reading "graph()" has been inserted into the expression field, if this is the first time the sketch graph has been used for this equation. You must insert, between the brackets, the name of the independent (x-axis) variable or other expression. If you have already entered a graph() expression in the equation dialogue box, then the opportunity is given to edit the previously-drawn curve. It is not possible to use two different graph() functions in the same expression.

Drawing the curve

The curve is constructed by moving the mouse into the graph pad, holding the mouse button down, then moving the mouse from left to right (or from right to left) along the desired path. You are unlikely to get it right first time. If a line segment ends up in the wrong place, just try again. For finer control, you can click on a vertical line at the required point: the ends of the two line segments meeting at this line will then jump into position.

Scaling the axes

The two edit fields along each axis (Min and Max) are used to indicate the range of each axis. You click in the edit field, then enter a value that is appropriate to the lower and upper limits respectively of the variable in the relationship. Thus, if the independent variable were temperature, and we were modelling a site in Africa, then the x-axis might be scaled to be between 10 and 40 (the units of degrees Celsius are implicit). When you change the scaling of an axis, the actual sketch stays the same, so the behaviour of the function is changed to correspond to the new axis scaling.

By default, "Min" and "Max" are 0 and 100 respectively on both axes. It is not permissible to leave any entry blank.

Current position

These two edit fields display the current position of the mouse pointer in the graph area whilst dragging the curve. You can use this display to exercise finer control over the curve. It is also possible to enter values directly into these boxes, copying values from a table for example. After entering the Y-value, press the return or enter key on the keyboard to have the graph re-drawn to include the new pair of values.

If you want exact values for all the points in the graph, click the "Edit as table" button, and enter them with the keyboard, or by cutting and pasting.


Between points

The normal behaviour of the graph function is to interpolate between the two nearest defined y values to get the function's result. Alternatively you can select 'Round' here, in which case you get the exact y value for the nearest defined argument value. The shape of the sketch graph changes to illustrate the actual values returned.

Out of range

There are three options to determine what happens if the independent variable falls outside the range scaled in this window, whilst the model is running.

  • truncate: this option extends the curve horizontally to the left and to the right.
  • extrapolate: this option extends the curve along a line with the same slope as the first and the last segment.
  • wraparound: this option joins up the left and the right ends of the curve. It is useful for defining repeating functions like sin(x).

X axis resolution

Clicking on the right-arrow button doubles the number of vertical lines in the graph pad area; clicking on the left-arrow button halves the number of lines. The higher the resolution, the greater the number of line segments making up the curve, and the smoother the curve is.

Altering function while model is running

The behaviour of the sketch can be altered while the model is running, using the edit sketch helper. This does not affect the version of the function in the saved model.

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