Look at the example that shipped with Labview: Cont Acq&Chart (immediate).vi. You can find it (in LabView 6.1) by selecting Help >> Find Examples >> Search >> Type a keyword to find: chart.
You can change the format of the X Scale to show a time format by right clicking on the chart, then selecting X Scale >> Formatting >> Format >> Time&Date. You can then select details about the Time and Date format (including changing Custom Date Format to Date Unused if you don't want to show the date). You can also on the Formatting page set a Xo to set your initial time, depending on what your data looks like.
You can also programmatically set the format and Xo using a Property node. (Or an Attribute node in version 5.x). Right click on chart, select Cr
eate >> Property node. Then, on diagram, right click on the property node and select Change to Write. Right click again and select Properties >> X Scale >> Format&Precision >> Format or Properties >> X Scale >> Range >> Minimum. If your data starts at t=0 but you want to start time when you start taking your data, use the Get Date/Time in Seconds function from the Time&Dialog palette and wire that output to a write property node for XScale.Minimum. It's probably easier to set the format non-programmatically (right click on the chart on the front panel) and set Xo = XScale.Minumum programmatically.