use the pyqtgraph import convention

mdi.py

@@ -1,7 +1,7 @@
 from PyQt5.QtCore import Qt
 from PyQt5.QtWidgets import QMdiArea, QMdiSubWindow, QAction
 from PyQt5.QtGui import QPainter
-from pyqtgraph import PlotWidget
+import pyqtgraph as pg
 
 import assets
 from theme import MDI_BKG, pg_plot_style
@@ -74,7 +74,7 @@ class MDISubPlot(QMdiSubWindow):
         data = zip(*data)
         style = pg_plot_style()
 
-        plt = PlotWidget()
+        plt = pg.PlotWidget()
         self.plot = plt.plot(*data, **style)
         self.setWidget(plt)