5: X-Y Plots

• Linegraphs: x-y graphs with data connected by line segments
• Scattergraphs: x-y graphs with data indicated by symbols
• Formula Plots:

5.1: Linegraphs

This plots a simple linegraph:

# Example 1 -- Linegraph using data in a separate file open example1.dat # Open the data file read columns x y # Read (x,y) columns draw curve # Draw data curve draw title "Example 1" # Title above plot

Here's what the command lines mean:

• The first line is a comment. Anything to the right of a hash-mark `#' is considered to be a comment. (This symbol is also called a "pound".)

• The second line is blank. Gri ignores blank lines between commands.
• `open example1.dat' tells Gri to open the indicated file (in the current directory) as an input data file. You can specify files outside the current directory by using conventional unix-shell pathnames (e.g., `open ~/data/TS/section1/T_S.dat' or `open ../data/file.dat'). You can even use "synonyms" (see Synonyms.) in filenames, as in `open \BASENAME.dat'.
• `read columns x y' tells Gri to start reading columnar data, the first column being `x', the second `y'. `x' and `y' are predefined names for whatever ends up on the horizontal and vertical axes.

  The number of data needn't be specified. Gri reads columns until a blank line or end-of-file is found. You can tell Gri how many lines to read with a command like `read columns 10 x y'. Multiple datasets can reside within one file; provided that they are separated by a single blank line, Gri can access them by multiple `read' commands.

  Like C, Gri expects numbers to be separated by one or more spaces or tabs. Commas are not allowed. If the columns were reversed, the command would be `read columns y x'. If there were an initial column of extraneous data, the command would be `read columns * x y', or `read columns x=2 y=3' (see Read Columns).

• `draw curve' tells Gri to draw a curve connecting the points in the `x' and `y' columns. A nice scale will be selected automatically. (You can change this or any other plot characteristics easily, as you'll see later.)
• `draw title' tells Gri to write the indicated string centered above the plot. The title must be enclosed in quotes.
• `quit' tells Gri to exit.

Gri will draw axes automatically, and pick its own scales.

If you wish to draw several curves which cross each other, you should try using `draw curve overlying' instead of `draw curve'. This will make it easier to distinguish the different curves.

5.2: Scattergraphs

To get a scattergraph with symbols at the data points, substitute `draw symbol' for `draw curve'. Both symbols and a curve result if both `draw curve' and `draw symbols' are used. See see Getting More Control for an example.

By default, the symbol used is an x. To get another symbol, use a command like `draw symbol 0' or `draw symbol plus'.

To change the symbol size from the default of 0.2 cm use commands like `set symbol size 0.1' to set to 1 mm (see Set Symbol Size).

5.2.1: Coding data with symbols

5.2.2: Drawing a symbol legend

set x axis -1 5 1 set y axis -1 5 1 read columns x y z 0 0 0 1 1 1 2 2 2 3 3 3 draw symbol # Legend .leg_x. = {rpn ..xmargin.. ..xsize.. + 1 +} .leg_y. = {rpn ..ymargin.. ..ysize.. 4 / +} draw symbol legend 0 "Foo" at .leg_x. .leg_y. cm .leg_y. += {rpn "M" ascent 2 *} draw symbol legend 1 "Bar" at .leg_x. .leg_y. cm .leg_y. += {rpn "M" ascent 2 *}

5.2.3: Coding data with symbol colors

5.3: Formula Plots

1. Use the system yourself. Do as in this example:

   open "awk 'BEGIN{for(i=0;i<3.141;i+=0.05)\ {print(i,cos(i))}}' |" read columns x y close draw curve

2. Let Gri calculate things for you

   The simplest is to let Gri calculate things for you with the `create columns from function' command (see Create). The command assumes that you have defined the synonym called `\function' which defines `y' in terms of `x'.

   Gri uses the program `awk' to create the columns, and cannot work without it.

   Here is an example of using `create columns from function':

   show "First 2 terms of perturbation expansion" set y axis name horizontal set y name "sea-level" set x name "$\omega$t" \b = "0.4" # perturbation parameter b=dH/H \xmin = "0" \xmax = "6.28" \xinc = "3.14 / 20" \function = "cos(x)" set x axis \xmin \xmax create columns from function draw curve draw title "SOLID LINE \function" \function = "(cos(x)+\b/2*(1-cos(2*x)))" create columns from function set dash 1 draw curve draw title "DASHED LINE \function" draw title "b = \b"

   Here's another example, in which the curve `y = 1/(\int + \sl*x)' is drawn through some data. Note how `sprintf' is used to set `\xmin' and `\xmax' using the scales that Gri has determined in reading the data.

   open file.data read columns x y close draw symbol bullet \int = "-0.1235" \sl = "0.003685" sprintf \xmin "%f" ..xleft.. sprintf \xmax "%f" ..xright.. \function = "1/(\int + x * \sl)" create columns from function draw curve