Working version!

Had to get the canvas object to draw_idle() in order for it
to be refreshed. Now it works great! I might tweak the step size
for the sliders, as 0.1 can end up with weird, not quite complete
plots.

spirographs.glade

@@ -136,6 +136,7 @@
         <child>
           <object class="GtkViewport" id="plotViewport">
             <property name="visible">True</property>
+            <property name="app-paintable">True</property>
             <property name="can-focus">False</property>
             <child>
               <placeholder/>

spirographs.py

@@ -24,9 +24,19 @@ class spirographs:
         self.distance = 4
         self.highestTheta = (np.lcm(self.smallRadius, self.bigRadius)/self.bigRadius) * 2 * math.pi
         self.stepSize = self.highestTheta / 4096
+        # update initial slider positions
+        bigRadiusAdjustment = builder.get_object('bigRadiusAdjustment')
+        smallRadiusAdjustment = builder.get_object('smallRadiusAdjustment')
+        distanceAdjustment = builder.get_object('distanceAdjustment')
+        bigRadiusAdjustment.set_value(self.bigRadius)
+        smallRadiusAdjustment.set_value(self.smallRadius)
+        distanceAdjustment.set_value(self.distance)
+        # create initial plot
         self.recalcPoints()
         self.showPlot()
 
+
+
     def calcEpiX(self, theta):
         return ((self.bigRadius + self.smallRadius) * math.cos(theta)) - (self.distance * math.cos(((self.bigRadius + self.smallRadius)/(self.smallRadius))*theta))
 
@@ -67,20 +77,24 @@ class spirographs:
     def showPlot(self):
         viewport = builder.get_object('plotViewport')
         self.plotFigure = Figure(figsize=(5, 4), dpi=100)
-        self.subPlot1 = self.plotFigure.add_subplot()
-        #self.subPlot1.title = f"Epichondroid of {self.bigRadius}, {self.smallRadius}, {self.distance}"
-        self.subPlot1.plot(self.epiX, self.epiY)
-        self.subPlot2 = self.plotFigure.add_subplot()
-        #self.subPlot2.title = f"Hypochondroid of {self.bigRadius}, {self.smallRadius}, {self.distance}"
-        self.subPlot2.plot(self.hypoX, self.hypoY)
+        self.epiPlot, self.hypoPlot = self.plotFigure.subplots(1, 2)
+        self.epiPlot.set_title(f"Epichondroid of {self.bigRadius}, {self.smallRadius}, {self.distance}")
+        self.epiPlot.plot(self.epiX, self.epiY)
+        self.hypoPlot.set_title(f"Hypochondroid of {self.bigRadius}, {self.smallRadius}, {self.distance}")
+        self.hypoPlot.plot(self.hypoX, self.hypoY)
 
         self.canvas = FigureCanvas(self.plotFigure)
         self.canvas.set_size_request(800, 600)
         viewport.add(self.canvas)
 
     def updatePlot(self):
-        self.subPlot1.plot(self.epiX, self.epiY)
-        self.subPlot2.plot(self.hypoX, self.hypoY)
+        self.epiPlot.clear()
+        self.hypoPlot.clear()
+        self.epiPlot.set_title(f"Epichondroid of {self.bigRadius}, {self.smallRadius}, {self.distance}")
+        self.hypoPlot.set_title(f"Hypochondroid of {self.bigRadius}, {self.smallRadius}, {self.distance}")
+        self.epiPlot.plot(self.epiX, self.epiY)
+        self.hypoPlot.plot(self.hypoX, self.hypoY)
+        self.canvas.draw_idle()
 
 builder = Gtk.Builder()
 builder.add_from_file("spirographs.glade")