QGIS » QGIS Application

I am using QGIS to visual health status of elderly in an elderly home, whether they have medical devices or not is one attribute that I am trying to visualize.

I have create the map using mapwindows, and use R to create a tab-delimited txt file to carry the medical device information (7 columns, 6 columns of 0 or 1, and one uid that will be used to join the bed layer in QGIS.

I used the diagram tab in the properties menu of the bed layer, and pie-chart is being used for visualization: if the elderly have 3 devices, the pie will be presented with 3 parts with 3 different colours. And strange things are visualized.

A majority of elderly do not have any devices (i.e. that should be straight zeros for the 6 column in the txt file). I support no pie chart should be visualized, but QGIS still show a solid pie using the last attribute I have inserted in the diagram tab of the properties menu.

I have tried to insert a csvt file to explicitly state that those medical device columns are integer, real, but it still fails. QGIS visual an attribute with value 0 as 1, if all of the attributes used for plotting are straight zeros.

Can anyone help?

Thanks.

The question was asked at GIS stackexchange: http://gis.stackexchange.com/questions/16194/strange-behaviour-of-piechart-function-of-qgis