Plotting Lines☆☆☆

The behavior of a PLOT LINES statement is described using the notion of 'beams'.
The state of a beam is 'on' or 'off'. If the beam moves when it is on, it draws a line.
The beam is set to be off when a program starts.

`PLOT LINES: x,y
` moves the beam to (x,y), and let it off.
`PLOT LINES: x,y ;
` moves the beam to (x,y), and let it on.
`PLOT LINES
` let the beam off.
`PLOT LINES: x1, y1 ; x2, y2 ; ･･･ ; xn , yn
` draws the polygonal line through the points specified, and then let the beam off.

[supplement]
ISO standard demands that beams shall be off on the following.
(1) Before executing of any other PLOT or GRAPH statement than PLOT LINES.
(2) Before executing of any GET, LOCATE, MAT PLOT, MAT GRAPH, MAT GET, or MAT LOCARE statement.
(3) Before executing of SET WINDOW, SET VIEWPORT, SET DEVICE WINDOW, or SET DEVICE VIEWPORT.
(4) Before and after the invocation of a PICTURE definition.
When you wish to enable (1) , (2) and (4), execute
`SET BEAM MODE "RIGOROUS"`

`SET LINE COLOR numeric_expression
` sets the color index that is used for line plotting.
The default line color is 1.
`SET LINE STYLE numeric_expression
` sets the line style that is used for line plotitng.
The line styles are
1 for solid, 2 for dashed, 3 for dotted, 4 for dashed-dotted.
The default line style is 1.

`ASK LINE COLOR Numeric_Variable
` assigns the current line color.
`ASK LINE STYLE Numeric_Variable
` assigns the current line style.
`ASK MAX LINE STYLE Numeric_Variable
` assigns the largest line style, 4 on this version.

Original Enhancements
`SET LINE WIDTH numeric_expression
` assigns the line width multiplier to the normal width.
`PLOT BEZIER: x1, y1 ; x2, y2 ; x3, y3; x4, y4
` draws a Bézier curve of terminal points (x1, y1)，(x4, y4), and control points (x2 , y2), (x3, y3).
`SET BEAM MODE "IMMORTAL"
` The beam shall remain on the execution of any GET, LOCATE, GRAPH or PLOT statement except PLOT LINES, or on any invocation of a PICTURE definition.
`SET BEAM MODE "RIGOROUS"
` The beam shall be off according to the Full BASIC standard.