enhance/add histogram sample (#137)

Author: Superjomn

visualdl/server/lib.py

@@ -146,7 +146,7 @@ def get_histogram_tags(storage):
     return get_tags(storage, 'histogram')
 
 
-def get_histogram(storage, mode, tag):
+def get_histogram(storage, mode, tag, num_samples=200):
     with storage.mode(mode) as reader:
         histogram = reader.histogram(tag)
         res = []
@@ -171,7 +171,22 @@ def get_histogram(storage, mode, tag):
                     [instance.left(),
                      instance.right(),
                      instance.frequency()])
-        return res
+        if len(res) < num_samples:
+            return res
+
+        # sample some steps
+        span = float(len(res)) / (num_samples - 1)
+        span_offset = 0
+        data_idx = 0
+
+        sampled_data = []
+        data_size = len(res)
+        while data_idx < data_size:
+            sampled_data.append(res[data_size - data_idx - 1])
+            span_offset += 1
+            data_idx = int(span_offset * span)
+        sampled_data.append(res[0])
+        return sampled_data[::-1]
 
 
 def retry(ntimes, function, time2sleep, *args, **kwargs):