Plots and sliders

Unlike the majority of system dynamics programs, Minsky enables the simulation itself to be dynamic: graphs are updated dynamically, and the model’s parameters can be varied as a simulation runs to see what happens. This is the reason that I placed Figure 24’s parameters at the top of the diagram: once plots are added, we will vary the parameters during a simulation.

Plots are added to the canvas using either the plot widget on the widget bar, or by pressing the @ key while on the Wiring canvas. This will create a default plot as shown in Figure 27.

Figure 25: The default Plot, with its stretch arrows highlighted

The key features of a plot are:

• The input ports: 4 on each Y axis and 8 on the X-axis;
• The Y-axis ports support up to 4 inputs on each Y-axis;
• The 8 input ports on the X-axis enable XY plots—up to 8 per plot. With nothing wired to the X-axis, the default input is simulation time, so that the plot is a time series graph. With an input on a given color on a Y-axis and the same on the X-axis, an XYplot is created;
• The axis max/min entries—shown as slightly tilted inputs with the same color on each axis (Red on the Y1 axis, Black on the X, Green on the Y2 axis).

Figure 26 shows how to use these features (there will be considerable improvements to the appearance and controls for plots in future releases).

Figure 26: Illustrating the elements of a plot

Variable parameters and dynamically updated plots enable a “what if?” approach to modelling. Figure 27 illustrates this by varying all 4 parameters at different times, and seeing what the impact was. Briefly, increasing credit as a percentage of GDP caused the money supply to grow and the economy to expand; increasing velocity caused the debt to GDP ratio to fall; the other two controls had largely no impact.

Figure 27: Simple model with plots and varying parameters

Parameter values can be altered in two ways:

1. By clicking on the black dot on the top of a parameter and dragging it right (to increase its value) or left (to decrease it); and
2. Pressing the up-arrow or right-arrow (to increase its value) or down-arrow or left-arrow (to decrease its value) when the mouse is hovering over the parameter.

The range of movement of the parameter’s value is set on the parameter definition popup—see Figure 28. Slider Bounds: Max and Slider Bounds: Min determine the parameter’s upper and lower bounds, while Slider Step Size determines how much the parameter’s value varies for each mouse movement or key press.

Figure 28: The Edit parameter popup