Gnuplot按数据值设不同颜色时，平滑曲线功能异常

云原生阿岚

2026-7-7

解决Gnuplot平滑温度曲线后颜色区分失效及报错问题

问题背景

原始Gnuplot配置可实现温度曲线按20℃分界着色（低于20℃为蓝色，≥20℃为红色），但为温度曲线添加smooth bezier平滑后，颜色区分消失，同时出现如下报错：

"minimon.gp" line 24: warning: extra columns ignored by smoothing option
"minimon.gp" line 29: warning: extra columns ignored by smoothing option
"minimon.gp" line 30: warning: extra columns ignored by smoothing option
"minimon.gp" line 30: warning: extra columns ignored by smoothing option
"minimon.gp" line 33: internal error : STRING operator applied to undefined or non-STRING variable

原因分析

Gnuplot的smooth操作默认仅处理x轴和y轴数据列，会忽略用来指定颜色的额外列，导致颜色变量丢失，进而触发报错。

解决方法

通过嵌套plot命令，先对温度数据执行平滑处理，再基于平滑后的温度值判断颜色。修改后的完整配置代码如下：

set datafile separator ","
set title "CO2,  Temp"
set key bottom right

set linetype 11 linecolor rgb "red"
set linetype 12 linecolor rgb "blue"

unset grid
set grid x
set grid y2

set ylabel "ppm"
set y2label "°C"

set xdata time
set timefmt "%Y-%m-%d %H:%M:%S"
set format x "%H:%M"  

set y2tics 
set y2range [15:30]  # range for temp


plot "minimonlog.csv" using 1:2 with lines smooth bezier axes x1y1 title "Co2", \
     "<plot 'minimonlog.csv' using 1:3 smooth bezier" using 1:2:($2 < 20 ? 12 : 11) linecolor variable with lines axes x1y2 title "Temp" 

done = 0
bind all 'd' 'done = 1'
while(!done) {
    replot
    pause 5      # waiting time in seconds
}