The HTBasic graphics system is based on the Cartesian coordinate system. This system uses a pair of values to define the location of each point in a graph relative to the origin at (0,0). The first value specifies how far the point is to the right of the origin and the second value specifies how far the point is above the origin. Negative values specify locations to the left of or below the origin. The horizontal line passing through the origin is called the X axis and a vertical line passing through the origin is called the Y axis.
The default origin, (0,0), is the lower left corner of the display screen. The default top vertical value is 100. The default right horizontal value depends upon the display aspect ratio and is usually about 148.
Let us now examine the simple graphic statements used to control the pen movement to generate graphic lines. They are the MOVE, DRAW, PLOT, and PENUP statements.
