Don't update scan chart data until there's actual data

[argilo]

If the scan chart is open prior to the first scan being completed, an uncaught error occurs:

Uncaught Error: Source data is missing a component at (0,1)!
    convertColumnsToData http://localhost:5000/static/js/c3.min.js:2
    loadFromArgs http://localhost:5000/static/js/c3.min.js:2
    load http://localhost:5000/static/js/c3.min.js:2
    redraw_scan_chart http://localhost:5000/static/js/scan_chart.js:80
    <anonymous> http://localhost:5000/:213
    jQuery 8
    <anonymous> http://localhost:5000/:199
    emit http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    emit http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    emit http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    emit http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    emit http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    emit http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    onreadystatechange http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    n http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    emit http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    onreadystatechange http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    n http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    emit http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    value http://localhost:5000/static/js/socket.io.min.js:6
    onreadystatechange http://localhost:5000/static/js/socket.io.min.js:6
    jQuery 2
    <anonymous> http://localhost:5000/:106
    jQuery 13
c3.min.js:2:122014

This occurs because the scan_chart_* variables are overwritten, even though get_scan_data has not provided any data:

radiosonde_auto_rx/auto_rx/autorx/templates/index.html

Lines 199 to 207 in d784d83

	$.getJSON("get_scan_data", function(data){
	// Load the data into our data stores.
	scan_chart_spectra.columns[0] = ['x_spectra'].concat(data.freq);
	scan_chart_spectra.columns[1] = ['Spectra'].concat(data.power);
	scan_chart_peaks.columns[0] = ['x_peaks'].concat(data.peak_freq);
	scan_chart_peaks.columns[1] = ['Peaks'].concat(data.peak_lvl);
	scan_chart_threshold.columns[0] = ['x_thresh'].concat([data.freq[0],data.freq[data.freq.length-1]]);
	scan_chart_threshold.columns[1] = ['Threshold'].concat([data.threshold+autorx_config.snr_threshold,data.threshold+autorx_config.snr_threshold]);
	scan_chart_latest_timestamp = data.timestamp;

The get_scan_data response looks like so:

{
    "freq": [],
    "power": [],
    "peak_freq": [],
    "peak_lvl": [],
    "timestamp":
    "No data yet.",
    "threshold": 0
}

Line 205 accesses non-existent array indices in data.freq, thereby setting the x coordinates of the threshold line to undefined.

To avoid this, we can skip processing if data.freq is empty, which leaves the placeholder data displayed instead.