third tentative fix for line chart field and topic name

src/LineChart/RosLineChart.js

@@ -86,7 +86,8 @@ class RosLineChart extends Component {
              * Extract a field from a ros message and add it to the state, for plotting.
              * 
              * If there are already maxPoints in storage, we remove the oldest data point.
-             * @param {string} field path to the field to be extracted from incoming messages
+             * @param {Object} curve has two entries, `field`, path to the field to be extracted from incoming messages
+             *  and `topic` the ROS topic from which we extract the field.
              * @param {integer} maxPoints mupper limit on the number of data entries for the plot
              * @param {Object} message messages from a ROS topic
              */
@@ -94,13 +95,14 @@ class RosLineChart extends Component {
                 this.setState((state) => {
                     let new_data = this.state.data;
                     const field = curve.field;
-                    if (!(field in new_data)) {
-                        new_data[field] = []
+                    const data_field = curve.topic+field;
+                    if (!(data_field in new_data)) {
+                        new_data[data_field] = []
                     }
-                    if (new_data[field].length >= maxPoints) {
-                        new_data[field].shift();
+                    if (new_data[data_field].length >= maxPoints) {
+                        new_data[data_field].shift();
                     }
-                    new_data[curve.topic+field] = this.state.data[curve.topic+field].concat([{"y": property(field)(message)}])
+                    new_data[data_field] = this.state.data[data_field].concat([{"y": property(field)(message)}])
                     return {
                         data: new_data,
                     }