Changes between Initial Version and Version 1 of Charts

Timestamp:

Sep 8, 2006, 5:17:12 PM (18 years ago)

Author:

andy@…

Comment:

--

Charts

@@ +1 @@
+This describes a simple way to create dynamic bitmap charts in Django.  
+
+!ReportLab's PDF library is well known, but less people are aware that it contains a
+mature graphics package able to create most kinds of businss charts, as well as custom graphical widgets.
+We've been serving GIF and PNG charts for many years.  Doing this within Django has turned out to be trivial; here's an example.
+
+= Installation =
+You will need the !ReportLab library from http://www.reportlab.org/
+
+Also make sure you have compiled the ''_rl_accel'' and ''_renderPM'' packages.  The latter
+is a sophisticated bitmap renderer with anti-aliasing, font handling and so on.
+ReportLab's download page has a checker script to tell you what extensions
+are installed.
+
+= Creating a chart =
+The reportlab/graphics framework provides a Drawing object and many Shape objects,
+which include primitive shapes, and high-level objects like charts and legends.  You
+can easily write your own too if you want to make dynamic bitmap buttons or fancy
+dashboard apps.
+
+The usual pattern we follow is to create a Drawing class with all of your widgets, 
+some sensible default data, and any visual settings you want.  Here is such a class,
+which has a bar chart and a string for the chart title.
+
+{{{
+#    mycharts.py  
+from reportlab.graphics.shapes import Drawing, String
+from reportlab.graphics.charts.barcharts import HorizontalBarChart
+
+class MyBarChartDrawing(Drawing):
+    def __init__(self, width=400, height=200, *args, **kw):
+        apply(Drawing.__init__,(self,width,height)+args,kw)
+        self.add(HorizontalBarChart(), name='chart')
+
+        self.add(String(200,180,'Hello World'), name='title')
+
+        #set any shapes, fonts, colors you want here.  We'll just
+        #set a title font and place the chart within the drawing
+        self.chart.x = 20
+        self.chart.y = 20
+        self.chart.width = self.width - 20
+        self.chart.height = self.height - 40
+
+        self.title.fontName = 'Helvetica-Bold'
+        self.title.fontSize = 12
+        
+        self.chart.data = [[100,150,200,235]]
+        
+
+if __name__=='__main__':
+    #use the standard 'save' method to save barchart.gif, barchart.pdf etc
+    #for quick feedback while working.
+    MyBarChartDrawing().save(formats=['gif','png','jpg','pdf'],outDir='.',fnRoot='barchart')
+}}}
+
+
+Paste this into a file and execute it; you should get 4 chart files written to 
+disk in different formats.
+
+= Integrating into Django =
+
+Now we add a view to our ''views.py''.  This will examine the request
+for any dynamic parameters, since there's not much point serving
+a chart that doesn't vary.  We'll allow the user to pass in 4
+things as GET or POST parameters:  a title, a comma-separated list of numbers,
+and the overall width and height of the image.  Everything has a default in
+our Drawing class anyway, so we only pass through parameters which are
+present.
+
+You then ask the Drawing to render itself to your favourite bitmap format
+and generate a response with the right content-type
+
+{{{
+def barchart(request):
+
+    #instantiate a drawing object
+    import mycharts
+    d = mycharts.MyBarChartDrawing()
+
+    #extract the request params of interest.
+    #I suggest having a default for everything.
+    if request.has_key('height'):
+        d.height = int(request['height'])
+    if request.has_key('width'):
+        d.height = int(request['width'])
+    
+    if request.has_key('numbers'):
+        strNumbers = request['numbers']
+        numbers = map(int, strNumbers.split(','))    
+        d.chart.data = [numbers]   #bar charts take a list-of-lists for data
+
+    if request.has_key('title'):
+        d.title.text = request['title']
+  
+
+    #get a GIF (or PNG, JPG, or whatever)
+    binaryStuff = d.asString('gif')
+    return HttpResponse(binaryStuff, 'image/gif')
+}}}
+
+Finally, you need a URL mapping.  In this case I have added this:
+
+{{{
+    (r'^charts/bar/$', 'myapp.views.barchart'),
+
+}}}
+
+Now you can start Django, point your browser at the URL with no arguments,
+and should see the chart.  Then try out a few parameters such as
+  http://localhost:8000/charts/bar?title=awesome
+and watch it redraw.
+
+= Tips =
+The above approach will let you drive a chart by generating
+img tags with all parameters embedded in them.   This will also let the world 
+use you as a chart server, so you might want to implement some authentication
+on the chart requests.    
+
+Very often you are plotting data which mostly exists on the server side.
+In this case it's inefficient to encode it all in a long URL in the HTML 
+then have this posted back to make the chart.  For example,
+if you run an ISP and are plotting server uptime statistics, presumably
+the URL or parameters would encode the customer and server ID, and you'd 
+look up all the numeric data or that chart in your view code in a database query.
+
+If you cannot always create a sensible chart from the input, it
+may be a good idea to create an error handler which returns a rectangular 
+drawing with an error message, because this will usually be called inside an <img> tag
+and the browser won't display any HTML error messages if it was expecting an image.
+
+= Learning More =
+The available chart types, widgets and properties are covered in the two
+Graphics manuals on this page.  Enjoy!
+   http://www.reportlab.org/os_documentation.html
+
+If you have questions about the charts rather than the Django integration, 
+ask on the reportlab-users list:
+  http://two.pairlist.net/mailman/listinfo/reportlab-users