Comprehensive Guide to Gnuplot Line Types: From Basic Concepts to Advanced Applications

Fundamental Concepts of Line Types

In Gnuplot, line type definition involves multiple key parameters. The linetype parameter controls both the visual style of lines and, by default, determines line color unless explicitly overridden by linecolor. User script examples demonstrate this mechanism:

set style line 1 lc rgb '#0060ad' lt 1 lw 2 pi -1 ps 1.0
set style line 2 lc rgb '#dd181f' lt 9 lw 2 pi -1 ps 1.0

Here, lt 1 and lt 9 correspond to different line types, while lc rgb overrides the default color settings.

Terminal Impact on Dashed Line Support

Dashed line rendering heavily depends on the selected terminal type. Some terminals like png (using libgd) completely lack dashed line support. Others like pngcairo support dashed lines but disable them by default, requiring explicit enabling via set termoption dashed or terminal-specific options.

Significant variations exist in dash patterns across different terminals. For instance, pngcairo and postscript terminals display completely different dash patterns. The test command provides visual comparison:

set terminal pngcairo dashed
set output 'test.png'
test
set output

Execution generates a test image containing all predefined line types, helping users verify actual rendering capabilities of their current terminal.

Major Changes in Version 5.0

Gnuplot version 5.0 introduced significant improvements in line type handling. The new dashtype parameter provides finer control over dash patterns. Users can employ predefined patterns:

plot x dashtype 2

Or define custom dash sequences:

plot x dashtype (3,5,10,5),\
     2*x dashtype '.-_'

Additionally, terminal options dashed and solid are ignored in version 5.0, with all lines defaulting to solid. To restore dashed lines, use loop configuration:

set for [i=1:8] linetype i dashtype i

The color management system was also enhanced, allowing selection between three different color sets via set colorsequence default|podo|classic command.

Analysis of Practical Application Issues

The user's issue of “only two out of four lines appearing dashed” typically stems from improper terminal configuration or version compatibility problems. In earlier versions, even with linetype set, if the terminal doesn't support or hasn't enabled dashed lines, rendering falls back to solid lines.

The problem described in the reference article further confirms the importance of terminal configuration. When users explicitly set lt 3 but rendering still shows lt 1, terminal driver limitations are likely the cause.

Cross-Version Compatibility Strategies

To ensure script compatibility across different Gnuplot versions, conditional configuration is recommended. Dynamically adjust parameter settings based on version detection:

if (GPVAL_VERSION >= 5.0) {
    set for [i=1:8] linetype i dashtype i
} else {
    set termoption dashed
}

For critical visualization needs, using terminals with advanced rendering capabilities like pngcairo or pdfcairo is advised, with thorough testing of actual output in production environments.

Debugging and Verification Techniques

When encountering abnormal line type display, systematic debugging is essential. First, use the test command to verify current terminal capabilities and confirm supported line type ranges. Then check version-specific settings, particularly differences between versions 4.6 and 5.0.

For complex data visualization projects, establishing standardized testing procedures across multiple terminals and version environments ensures rendering consistency and final output meets design expectations.