Fix log scale handling in magnitude spectrum function

Fix #169

Author: PierreRaybaut

CHANGELOG.md

@@ -6,6 +6,7 @@ See DataLab [roadmap page](https://datalab-platform.com/en/contributing/roadmap.
 
 🛠️ Bug fixes:
 
+* Fixed [Issue #169](https://github.com/DataLab-Platform/DataLab/issues/169) - Signal / Fourier analysis: magnitude spectrum feature does not work as expected with logarithmic scale enabled
 * Fixed [Issue #168](https://github.com/DataLab-Platform/DataLab/issues/168) - Average profile visualization: empty profile is displayed when the target rectangular area is outside the image area (this has been fixed upstream, in PlotPy v2.7.4, and so requires the latest version of PlotPy)
 
 ## DataLab Version 0.19.1 ##

cdl/algorithms/signal.py

@@ -119,7 +119,8 @@ def magnitude_spectrum(
     x1, y1 = fft1d(x, y)
     if log_scale:
         y_mag = 20 * np.log10(np.abs(y1))
-    y_mag = np.abs(y1)
+    else:
+        y_mag = np.abs(y1)
     return x1, y_mag