You can update the symbol in the chart xml. Each chart should have its own _data file associated with it. That is the file where the formatting is done.
For instance in the file DynamicWebCharts/Data/orders_Chart_daily_data2.php
you can update:
<tooltip><![CDATA[#x_label#<br>Revenue: $#val# ]]></tooltip>
to:
<tooltip><![CDATA[#x_label#<br>Revenue: £#val# ]]></tooltip>