python-control
diff --git a/‎control/grid.py‎
Lines changed: 45 additions & 29 deletions b/‎control/grid.py‎
Lines changed: 45 additions & 29 deletions
@@ -64,15 +64,18 @@ def __call__(self, transform_xy, x1, y1, x2, y2):
         return lon_min, lon_max, lat_min, lat_max
 
 
-def sgrid(ax=None):
-    """Draws continuous time damping and frequency grid"""
+def sgrid(fig=None, position=None):
+    """Creates a plot axis with an s-plane (continuous-time) grid
+    of constant damping factors and natural frequencies.
+    """
     # From matplotlib demos:
     # https://matplotlib.org/gallery/axisartist/demo_curvelinear_grid.html
     # https://matplotlib.org/gallery/axisartist/demo_floating_axis.html
 
-    if ax is None:
-        ax = plt.gca()
-    fig = ax.figure
+    if fig is None:
+        fig = plt.gcf()
+    if position is None:
+        position = (1, 1, 1)
 
     # PolarAxes.PolarTransform takes radian. However, we want our coordinate
     # system in degree
@@ -82,19 +85,24 @@ def sgrid(ax=None):
     # (min, max of the coordinate within the view).
 
     # 20, 20 : number of sampling points along x, y direction
-    sampling_points = 20
-    extreme_finder = ModifiedExtremeFinderCycle(
-        sampling_points, sampling_points, lon_cycle=360, lat_cycle=None,
-        lon_minmax=(90, 270), lat_minmax=(0, np.inf),)
+    n_points = 20
+    extreme_finder = ModifiedExtremeFinderCycle(n_points, n_points,
+                                                lon_cycle=360,
+                                                lat_cycle=None,
+                                                lon_minmax=(90,270),
+                                                lat_minmax=(0, np.inf))
 
     grid_locator1 = angle_helper.LocatorDMS(15)
     tick_formatter1 = FormatterDMS()
-    grid_helper = GridHelperCurveLinear(
-        tr, extreme_finder=extreme_finder, grid_locator1=grid_locator1,
-        tick_formatter1=tick_formatter1)
+    grid_helper = GridHelperCurveLinear(tr,
+                                        extreme_finder=extreme_finder,
+                                        grid_locator1=grid_locator1,
+                                        tick_formatter1=tick_formatter1
+                                        )
 
-    fig = plt.gcf()
-    ax = SubplotHost(fig, 1, 1, 1, grid_helper=grid_helper)
+    # TODO: Allow positions of type '211', '212' etc.
+    ax = SubplotHost(fig, position[0], position[1], position[2], 
+                     grid_helper=grid_helper)
 
     # make ticklabels of right invisible, and top axis visible.
     visible = True
@@ -124,27 +132,28 @@ def sgrid(ax=None):
 
     fig.add_subplot(ax)
 
-    # RECTANGULAR X Y AXES WITH SCALE
-    # par2 = ax.twiny()
-    # par2.axis["top"].toggle(all=False)
-    # par2.axis["right"].toggle(all=False)
-    # new_fixed_axis = par2.get_grid_helper().new_fixed_axis
-    # par2.axis["left"] = new_fixed_axis(loc="left",
+    ### RECTANGULAR X Y AXES WITH SCALE
+    #par2 = ax.twiny()
+    #par2.axis["top"].toggle(all=False)
+    #par2.axis["right"].toggle(all=False)
+    #new_fixed_axis = par2.get_grid_helper().new_fixed_axis
+    #par2.axis["left"] = new_fixed_axis(loc="left",
     #                                   axes=par2,
     #                                   offset=(0, 0))
-    # par2.axis["bottom"] = new_fixed_axis(loc="bottom",
+    #par2.axis["bottom"] = new_fixed_axis(loc="bottom",
     #                                     axes=par2,
     #                                     offset=(0, 0))
-    # FINISH RECTANGULAR
+    ### FINISH RECTANGULAR
 
-    ax.grid(True, zorder=0, linestyle='dotted')
+    ax.grid(True, zorder=0,linestyle='dotted')
 
     _final_setup(ax)
     return ax, fig
 
 
 def _final_setup(ax, axcolor='black', axlinestyle='-', 
                  axlinewidth=1, equal=True):
+    # Add axes labels and lines through the origin     
     ax.set_xlabel('Real')
     ax.set_ylabel('Imaginary')
     ax.axhline(y=0, linestyle=axlinestyle, color=axcolor, lw=axlinewidth,
@@ -163,12 +172,19 @@ def nogrid(ax=None, axlinestyle='-', axlinewidth=1):
     return ax, fig
 
 
-def zgrid(zetas=None, wns=None, ax=None):
-    '''Draws discrete time damping and frequency grid'''
-
-    if ax is None:
-        ax = plt.gca()
-    fig = ax.figure
+def zgrid(zetas=None, wns=None, fig=None, ax=None, position=None):
+    """Creates a plot axis with a z-plane (discrete-time) grid of
+    constant damping factors and natural frequencies.
+    """
+    if fig is None:
+        if ax is None:
+            ax = plt.gca()
+        fig = ax.figure
+    else:
+        # Create the axis if only figure is provided
+        if position is None:
+            position = (1, 1, 1)
+        ax = fig.add_subplot(position[0], position[1], position[2])
 
     # Constant damping lines
     if zetas is None: