Changing the starting values of the bars

To adjust the value the bars are starting at you need to set a `fromvalue` at
the `barpos` style. ... By default the bars start at the baseline not taking
into account its value at all.

!! When setting a `barpos` style explicitely, no other positioning style
instance is needed by the bar style and thus you can set other default values
for the `barpos` style. Note, that as soon as you explicitely set a barpos
style, you can also use other styles requesting a point position at the graph
(since the bar style also initializes a point position). Hence you could for
example replace the bar style in this example by a `graph.style.symbol`
instance.