improve empty spectrogram error message

fixes #13

Author: swharden

src/Spectrogram.Tests/Mel.cs

@@ -82,5 +82,15 @@ public void Test_Mel_Graph()
 
             plt1.SaveFig("mel1.png");
         }
+
+        [Test]
+        public void Test_SaveEmpty_Throws()
+        {
+            (int sampleRate, double[] audio) = WavFile.ReadMono("../../../../../data/cant-do-that-44100.wav");
+            int fftSize = 4096;
+            var spec = new Spectrogram(sampleRate, fftSize, stepSize: 500);
+            //spec.Add(audio);
+            Assert.Throws<InvalidOperationException>(() => { spec.SaveImage("empty.png"); });
+        }
     }
 }

src/Spectrogram/Spectrogram.cs

@@ -182,7 +182,10 @@ public void SaveBitmap(Bitmap bmp, string fileName) { }
 
         public void SaveImage(string fileName, double intensity = 1, bool dB = false, bool roll = false)
         {
-            string extension = System.IO.Path.GetExtension(fileName).ToLower();
+            if (ffts.Count == 0)
+                throw new InvalidOperationException("Spectrogram contains no data. Use Add() to add signal data.");
+
+            string extension = Path.GetExtension(fileName).ToLower();
 
             ImageFormat fmt;
             if (extension == ".bmp")