我想使用ns3中的GnuplotHelper来绘制结果。即使在运行NS3的默认示例seventh.cc时，也会出现以下错误:

../scratch/congestion.cc: In function ‘int main(int, char**)’:
../scratch/congestion.cc:173:1: error: ‘GnuplotHelper’ was not declared in this scope
 GnuplotHelper plotHelper;
 ^
../scratch/congestion.cc:180:1: error: ‘plotHelper’ was not declared in this scope
 plotHelper.ConfigurePlot ("Test","CongestionWindow vs. Time","Time (Seconds)","CongestionWindow","jpg");
 ^
../scratch/congestion.cc:181:81: error: ‘GnuplotAggregator’ has not been declared
 plotHelper.PlotProbe (probeName,probeTrace,"CongetionWindow","CongestionWindow",GnuplotAggregator::KEY_BELOW);

如果包含“gnuplot-helper.h”，则会出现额外的一行和相同的错误:

../scratch/seventh.cc:24:28: fatal error: gnuplot-helper.h: No such file or directory
compilation terminated.

我在Linux上安装了gnuplot。我该怎么办？如何声明Gnuplot助手？