[FEM] fix linear filter plot (#6999)

* [FEM] fix linear filter plot

the plot one can create from within the linear filter is at the moment detached from FreeCAD's main window.
This PR fixes it the same way as in https://github.com/FreeCAD/FreeCAD/commit/920e8e046

src/Mod/Fem/Gui/TaskPostBoxes.cpp

@@ -735,14 +735,19 @@ std::string TaskPostDataAlongLine::Plot() {
     auto xlabel = tr("Length", "X-Axis plot label");
     std::ostringstream oss;
     oss << "import FreeCAD\n\
+from PySide import QtCore\n\
 import numpy as np\n\
 from matplotlib import pyplot as plt\n\
+plt.ioff()\n\
 plt.figure(1)\n\
 plt.plot(x, y)\n\
 plt.xlabel(\"" << xlabel.toStdString() << "\")\n\
 plt.ylabel(title)\n\
 plt.title(title)\n\
 plt.grid()\n\
+fig_manager = plt.get_current_fig_manager()\n\
+fig_manager.window.setParent(FreeCADGui.getMainWindow())\n\
+fig_manager.window.setWindowFlag(QtCore.Qt.Tool)\n\
 plt.show()\n";
     return oss.str();
 }