tweak benchmark code

demos/bench_gridder.py

@@ -11,10 +11,10 @@ np.random.seed(40)
 N0s, a0s, b0s, c0s = [], [], [], []
 N1s, a1s, b1s, c1s = [], [], [], []
 
-for ii in range(1, 8):
+for ii in range(10, 23):
     nu = 1024
     nv = 1024
-    N = int(10**ii)
+    N = int(2**ii)
     print('N = {}'.format(N))
 
     uv = np.random.rand(N, 2) - 0.5
@@ -70,6 +70,8 @@ print('FFT shape', res.shape)
 plt.scatter(N0s, a0s, label='Gridder mr')
 plt.scatter(N1s, a1s, marker='^', label='NFFT')
 plt.legend()
+# no idea why this is necessary, but if it is omitted, the range is wrong
+plt.ylim(min(a0s+a1s), max(a0s+a1s))
 plt.ylabel('time [s]')
 plt.title('Initialization')
 plt.loglog()