Data Sorting Issues and Solutions in Gnuplot Multi-Line Graph Plotting

Problem Background and Phenomenon Analysis

When plotting multi-line graphs in Gnuplot, users often encounter situations where the graphical output doesn't match expectations. A typical case involves a data file ls.dat containing version numbers, removed counts, added counts, and modified counts, with the following structure:

# Gnuplot script file for "ls"
# Version       Removed Added   Modified
8.1     0       0       0
8.4     0       0       4
8.5     2       5       9
8.6     2       7       51
8.7     2       7       51
8.8     2       7       51
8.9     2       7       51
8.10    2       7       51
8.11    2       8       112
8.12    2       8       112
8.13    2       17      175
8.17    6       33      213

The user attempts to plot three lines using this command:

plot "ls.dat" using 1:2 title 'Removed' with lines,\
     "ls.dat" using 1:3 title 'Added' with lines,\
     "ls.dat" using 1:4 title 'Modified' with lines

The expected result is three increasing lines over time (version numbers), but the actual output shows abnormal connections between data points.

Core Problem Diagnosis

The root cause lies in how Gnuplot parses x-axis data. When using using 1:2, Gnuplot treats the first column as x-coordinates. In this case, the first column contains version numbers like "8.1", "8.4", etc. Gnuplot interprets these as floating-point numbers, causing "8.10" to be parsed as 8.1, which conflicts with "8.1" and disrupts the data order. This parsing error leads to incorrect line connections that don't properly reflect data trends.

Primary Solutions

Based on the best answer (Answer 1), there are two effective approaches:

Solution 1: Modify Data Format

Convert version numbers to uniform two-decimal format, e.g., change "8.1" to "8.01", "8.4" to "8.04", and so on. This ensures Gnuplot correctly parses numerical order. Modified data example:

8.01    0       0       0
8.04    0       0       4
8.05    2       5       9
8.06    2       7       51
8.07    2       7       51
8.08    2       7       51
8.09    2       7       51
8.10    2       7       51
8.11    2       8       112
8.12    2       8       112
8.13    2       17      175
8.17    6       33      213

The original plotting command will then produce correct multi-line graphs.

Solution 2: Use Integer Indices as X-Axis

A more straightforward method is to ignore the first column and use the natural row order as x-coordinates:

plot "ls.dat" using 2 title 'Removed' with lines, \
     "ls.dat" using 3 title 'Added' with lines, \
     "ls.dat" using 4 title 'Modified' with lines

Here, Gnuplot automatically uses integer sequences (1, 2, 3, ...) as x-coordinates, with y-coordinates from columns 2, 3, and 4 respectively. This avoids version number parsing issues but loses specific version labels.

Supplementary Techniques and Extensions

Referencing other answers, further optimization is possible:

Preserving Version Labels

As noted in Answer 2, the xtic() function maintains version labels while using integer indices:

plot 'ls.dat' using 4:xtic(1)

This command uses column 4 (Modified) as y-values, integer indices as x-axis, but displays version numbers from column 1 as tick labels. For multi-line graphs, each line needs separate handling:

plot 'ls.dat' using 2:xtic(1) title 'Removed' with lines, \
     'ls.dat' using 3 title 'Added' with lines, \
     'ls.dat' using 4 title 'Modified' with lines

Note: xtic(1) only needs specification in the first line; subsequent lines automatically inherit the same x-axis settings.

General Data Format Handling

Answer 3 reminds us to consider data file separators. For comma-separated CSV files, first set:

set datafile separator comma

Then use standard using commands. This setting also applies to other separators like tabs or spaces.

Deep Understanding of the using Command

The basic syntax of the using command is using x:y, where x and y are column indices. When only one parameter is specified, e.g., using 2, Gnuplot interprets it as the y-value, with x defaulting to the data row number. This flexibility allows users to choose coordinate systems as needed.

Practical recommendations:

1. For numerical x-data, ensure uniform formatting to avoid parsing ambiguities
2. For label-type x-data (e.g., version numbers, dates), consider using integer indices with xtic()
3. Always verify data sorting; use the print command to check values actually read by Gnuplot

Conclusion

Data sorting issues in Gnuplot multi-line graph plotting typically stem from x-axis data parsing anomalies. By modifying data formats or using integer indices, graphs can accurately reflect data trends. Combined with the xtic() function, meaningful labels can be preserved while maintaining correct ordering. Understanding Gnuplot's data processing mechanisms helps avoid similar issues and create more accurate data visualizations.