HighCharts and D3 are both data visualization tools that you can add to your website.
Highcharts is an open source Javascript library that provides all kinds of different charts you can use. It is supported by all modern browsers (even IE6+!) and mobile browsers. It does not require client-side plugins like Java or Flash. The HighCharts API allows you to update charts at any time after creation, so they can update as frequently as once per second.
See the demo page here. The demo page includes source code for each of the chart types, which is super useful!