Fix linechart for same field of different topics

There was a bug in the case where the same field was taken frop several topics. The data of each topic would be appended to a single line.

src/LineChart/RosLineChart.js

@@ -90,16 +90,17 @@ class RosLineChart extends Component {
              * @param {integer} maxPoints mupper limit on the number of data entries for the plot
              * @param {Object} message messages from a ROS topic
              */
-            function process_message(field, maxPoints, message) {
+            function process_message(curve, maxPoints, message) {
                 this.setState((state) => {
                     let new_data = this.state.data;
+                    const field = curve.field;
                     if (!(field in new_data)) {
                         new_data[field] = []
                     }
                     if (new_data[field].length >= maxPoints) {
                         new_data[field].shift();
                     }
-                    new_data[field] = this.state.data[field].concat([{"y": property(field)(message)}])
+                    new_data[curve.topic+field] = this.state.data[field].concat([{"y": property(field)(message)}])
                     return {
                         data: new_data,
                     }
@@ -111,7 +112,7 @@ class RosLineChart extends Component {
             if ('curves' in config) {
                 config.curves.forEach((curve) => {
                     const listener = new RosLib.Topic({ros : this.props.ros, name: curve.topic});
-                    listener.subscribe(process_message.bind(this, curve.field, config.maxPoints));
+                    listener.subscribe(process_message.bind(this, curve, config.maxPoints));
                     this.listeners.push(listener);
                 });
             }