[plot] addHistogram do not update an existing histogram
```
In [5]: plot = silx.gui.plot.Plot1D()

In [6]: plot.setVisible(True)

In [7]: plot.addHistogram(edges=[1,2,3], histogram=[1,2,3])
Out[7]: 'Unnamed histogram'

In [8]: plot.addHistogram(edges=[1,2,3], histogram=[1,5,3])
Out[8]: 'Unnamed histogram'
```
After the last command the histogram stays displayed with 1, 2, 3.
It is not the same behaviour as `addCurve`.

```
In [13]: plot.addCurve([1,2,3], [1,2,3])
Out[13]: 'Unnamed curve 1.1'

In [14]: plot.addCurve([1,2,3], [1,5,3])
Out[14]: 'Unnamed curve 1.1'
```
diff --git a/silx/gui/plot/items/histogram.py b/silx/gui/plot/items/histogram.py
index c3821bc25..b8cd4adf6 100644
--- a/silx/gui/plot/items/histogram.py
+++ b/silx/gui/plot/items/histogram.py
@@ -286,3 +286,5 @@ class Histogram(Item, AlphaMixIn, ColorMixIn, FillMixIn,
 
             self._histogram = histogram
             self._edges = edges
+
+        self._updated()
['silx/gui/plot/items/histogram.py']