Is it possible to have the representation like the one the heat-map like the figure below using QGIS? The way seems to use buffer options, but I do not know how. Does exist any plugin that helps? I am using QGIS1.8 on windows 7
QGIS has a HeatMap Plugin that will do you what you want. Find it under Plugins > Heatmap. There are some instructions here: http://qgis.spatialthoughts.com/2012/07/tutorial-making-heatmaps-using-qgis-and.html
HeatMap Plugin for QGIS 1.8 is available
The Heatmap plugin allows to create a heatmap from a point vector map. A heatmap is a raster map showing the density or magnitude of point related information. From the result “hotspots” can easily be identified. Activate the Heatmap plugin
First this core plugin needs to be activated using the Plugin Manager (see Section Loading a QGIS Core Plugin). After activation the heatmap icon heatmap can be found in the Raster Toolbar.
Select from menu View ‣ Toolbars ‣ Raster to activate the Raster Toolbar when it is not yet activated.
Since Qgis 2.8 heatmap can be rendered on live without any plugin. See the doc http://www.qgis.org/en/site/forusers/visualchangelog28/index.html?highlight=heatmap#feature-live-heatmap-renderer