DESCRIPTION

d.linegraph is a module to draw simple x,y line graphs (plots) based on numerical data contained in separate files.

Data format

The X and Y data files for the graph are essentially a column of numbers in each file, with one input number per line. The program expects that each X value will have a corresponding Y value, therefore the number of lines in each data input file should be the same. Essentially, the X data becomes the X axis reference to which the Y data is plotted as a line. Therefore, the X data should be a monotonically increasing progression of numbers (i.e. "1,2,3,..."; "0, 10, 100, 1000,..."; "...-5,-1,0,1,5..."). If multiple Y data files are used, the Y axis scale will be based on the range of minimum and maximum values from all Y files, then all Y data given will be graphed according to that Y scale. Therefore, if multiple Y data inputs are used with dissimilar units, the graph produced comparing the two will be deceptive.

File inputs

If the directory option is provided, the paths to files can (and should) be only relative paths to these files. While this is not recommended for scripting, it can be advantageous when typing the paths manually. For example when all files are stored in the directory /home/john/data, the user can provide the following in the command line:

d.linegraph directory=/home/john/data x_file=x.txt y_file=y1.txt,y2.txt

Managing colors

The user can specify the y_color option, the color_table option or just leave the defaults to influence the color of the plotted lines.

Colors specified by y_color option are used for drawing the lines in the graph. If multiple Y data files are used, an equal number of colors may be used to control the colors of the lines. Colors will be assigned to Y data in respect to the sequence of instantiation on the command line. It can be one of GRASS GIS named colors or the RGB values from 0-255 separated by colons (RRR:GGG:BBB).

Alternatively, the user can use the color_table option to specify one of the GRASS GIS predefined color tables.

By default, a series of colors will be chosen by the module if none are provided upon invocation. The order of default colors is red, green, violet, blue, orange, gray, brown, magenta, white, and indigo. The user is advised not to rely on the order of default colors but to either use the y_color or the color_table option to obtain predictable and reproducible results.

The color to be used for titles, axis lines, tics, and scale numbers is determined by the title_color option. The user can provide one of the GRASS GIS named colors (such as gray, white, or black) or use the GRASS GIS colon-separated format for RGB (RRR:GGG:BBB).

Titles, labels, and tics

The title option specifies the text for the title of the graph. It will be centered over the top of graph. The x_title option is a text to describe data for X axis. It will be centered beneath the graph. Default is no text unless there is a need for a unit descriptor determined by the d.linegraph module, then string such as "in hundreds" is generated. The y_title option is a text to describe data for Y axis. It will be centered beneath the X data description. Similarly, to the x_title option, default is no text unless there is a need for an auto-generated description. In the case of graphs with multiple lines (multiple inputs for Y axis), user may wish to use more specific text placement using the d.text or v.label programs.

NOTES

For historical reasons, the d.linegraph module accepts titles of more than one word where the underscore character ("_") is used to represent spaces (" "). For example "Census_data_1990" would be printed over the graph as "Census data 1990". The use of underscores is not necessary to use as long as the parameter is quoted in the command line. In general, use of underscores is not recommended and there is no need to use it at all in the GUI or when using d.linegraph in Python scripts.

The way the program locates and labels tic marks is less than perfect:
1) although distances between Y tics are proportional to the value, they are not proportional on the X axis;
2) decimal values between -1 and 1 can be printed on the X axis, but not on Y. (With respect to the later, the input for Y values can all be multiplied by a factor of 10 before graphing).

Depending on the user's needs, it might be easier or more appropriate to use a 3rd party tool such as xgraph, gnuplot, Matplotlib in Python, or R instead of d.linegraph. For a more general solution for plotting in GRASS GIS, the user is advised to use the d.graph module.

EXAMPLE

The following can be executed in Bash to create the input data for this example. The user can just create these files in a text editor, save them and specify path to them.

cat > x.txt <<EOF
1
3
4
6
9
EOF
cat > y1.txt <<EOF
50
58
65
34
27
EOF
cat > y2.txt <<EOF
10
20
35
50
45
EOF

The next command sequence creates a file plot.png in the current directory which is the drawing made by d.linegraph.

d.mon start=cairo output=plot.png width=400 height=400
d.linegraph x_file=x.txt y_file=y1.txt,y2.txt
d.mon stop=cairo

AUTHOR

Chris Rewerts, Agricultural Engineering, Purdue University

Last changed: $Date$