Dynamic Vertical Axis to Accommodate Trendline(s) in Google Line Chart

Question

I'm new to coding but over the last several months I've managed to fumble my way through creating a web site that utilises a Google Line Chart and embedded linear trendline to display historical Mean Sea Level and the rate of Mean Sea Level rise for various locations around New Zealand and the Pacific. Each location has it's own Google Line Chart with a linear trendline to show the rate of Mean Sea Level Change for a user selected period. I now want to extend the functionality of each Google Line Chart such that both a linear and polynomial trendline extend to the year 2120 (they currently only show up to the year 2018) even though the available data from which they are calculated uses observed data up to the year 2018. This will allow the user to predict the sea level height up to the year 2020. I realise this explanation may be confusing, so please see my web site www.sealevel.nz to see the existing charts which I hope will aid in understanding my problem.

Below is the code for the extended version of the chart that shows both a linear and second degree polynomial trendline with the x axis of the Google Line Chart now showing up the year 2120. My problem is that I need the y axis to adjust dynamically to show the entirety of both trendlines no matter which time period the user selects. For example if you select the years 1971 and 2018 from the date range slider, then both trendlines are cut off at the years 2017 (linear) and 2031 (polynomial) respectively. I need to be able to see both trendlines and their values up to the year 2120.

Please excuse my novice coding skills. My Code:

<script type="text/javascript" src="https://www.gstatic.com/charts/loader.js"></script>
<script src="https://unpkg.com/mathjs/dist/math.min.js"></script>
<script type="text/javascript">

google.load('visualization', 'current', {'packages':['controls','corechart']});
google.setOnLoadCallback(initialize);
function initialize() {
  var query = new google.visualization.Query('https://docs.google.com/spreadsheets/d/1vn1iuhsG33XzFrC4QwkTdUnxOGdcPQOj-cuaEZeX-eA/edit#gid=0');
  query.send(drawDashboard);
}
function drawDashboard(response) {
  var data = response.getDataTable();
//Asign units of 'mm' to data.
    var formatMS = new google.visualization.NumberFormat({
    pattern: '# mm'
  });
  // format into data mm..
  for (var colIndex = 1; colIndex < data.getNumberOfColumns(); colIndex++) {
    formatMS.format(data, colIndex);
  }
 var YearPicker = new google.visualization.ControlWrapper({
    'controlType': 'NumberRangeFilter',
    'containerId': 'filter_div',
    'options': {
      'filterColumnLabel': 'Year',
        'ui': {
       cssClass: 'filter-date',
          'format': { pattern: '0000' },
      'labelStacking': 'vertical',
      'allowTyping': false,
      'allowMultiple': false    
      }
    },
  });
  var MSLChart = new google.visualization.ChartWrapper({
    'chartType': 'LineChart',
    'containerId': 'chart_div',
    'options': {  
    'fontSize': '14', 
    'title': 'Timbucktoo Annual Mean Sea Level Summary',
        hAxis: {title: 'Year', format:'0000', maxValue: 2120},
        vAxis: {title: 'Height above Chart Datum (mm)', format:'0000'},
        'height': 600,
    chartArea: {height: '81%', width: '85%', left: 100},
    'legend': {'position': 'in', 'alignment':'end', textStyle: {fontSize: 13} },
    colors: ['blue'],
    trendlines: {
            0: {
                type: 'polynomial',
                degree: 2,
                color: 'green',
                visibleInLegend: true,
            },
            1: {
                type: 'linear',
                color: 'black',
                visibleInLegend: true,
            },
        },
        series: {
            0: { visibleInLegend: true },
            1: { visibleInLegend: false },
        },    
    },
    'view': {'columns': [0,1,2]}
  });

  var dashboard = new google.visualization.Dashboard(document.getElementById('dashboard_div')).
    bind(YearPicker, MSLChart).

  draw(data)
 </script>

Answer 1

first, I'm not sure why the chart would draw a trend line that isn't visible
which makes this a bit tricky, because we first have to draw the chart,
in order to find the min & max y-axis values.

but there are chart methods we can use to find the max value.

first, we get the chart's layout interface.

var chartLayout = MSLChart.getChart().getChartLayoutInterface();

since we're using a ChartWrapper, we have to get the chart from the wrapper (MSLChart.getChart()).

next, we use method getBoundingBox to find the min & max values of each line.

var yAxisCoords = {min: null, max: null};
var lineIndex = 0;
var boundsLine = chartLayout.getBoundingBox('line#' + lineIndex);
do {
  yAxisCoords.max = yAxisCoords.max || boundsLine.top;
  yAxisCoords.max = Math.min(yAxisCoords.max, boundsLine.top);
  yAxisCoords.min = yAxisCoords.min || (boundsLine.top + boundsLine.height);
  yAxisCoords.min = Math.max(yAxisCoords.min, (boundsLine.top + boundsLine.height));
  lineIndex++;
  boundsLine = chartLayout.getBoundingBox('line#' + lineIndex);
} while (boundsLine !== null);

then we use method getVAxisValue to determine what each y-axis value should be,
set the viewWindow on the y-axis, and re-draw the chart.

MSLChart.setOption('vAxis.viewWindow.max', chartLayout.getVAxisValue(yAxisCoords.max));
MSLChart.setOption('vAxis.viewWindow.min', chartLayout.getVAxisValue(yAxisCoords.min));
MSLChart.draw();

we do all this in a function.
we use a one time 'ready' event on the chart wrapper for the first calculation.
then again, on the chart.

google.visualization.events.addOneTimeListener(MSLChart, 'ready', filterChange);

function filterChange() {
  // get chart layout
  var chartLayout = MSLChart.getChart().getChartLayoutInterface();

  // get y-axis bounds
  var yAxisCoords = {min: null, max: null};
  var lineIndex = 0;
  var boundsLine = chartLayout.getBoundingBox('line#' + lineIndex);
  do {
    yAxisCoords.max = yAxisCoords.max || boundsLine.top;
    yAxisCoords.max = Math.min(yAxisCoords.max, boundsLine.top);
    yAxisCoords.min = yAxisCoords.min || (boundsLine.top + boundsLine.height);
    yAxisCoords.min = Math.max(yAxisCoords.min, (boundsLine.top + boundsLine.height));
    lineIndex++;
    boundsLine = chartLayout.getBoundingBox('line#' + lineIndex);
  } while (boundsLine !== null);

  // re-draw chart
  MSLChart.setOption('vAxis.viewWindow.max', chartLayout.getVAxisValue(yAxisCoords.max));
  MSLChart.setOption('vAxis.viewWindow.min', chartLayout.getVAxisValue(yAxisCoords.min));
  MSLChart.draw();
  google.visualization.events.addOneTimeListener(MSLChart.getChart(), 'ready', filterChange);
}

---

see following working snippet...
(when you run the snippet, click "full page" at the top right)

google.charts.load('current', {
  packages: ['controls']
}).then(initialize);

function initialize() {
  var query = new google.visualization.Query('https://docs.google.com/spreadsheets/d/1vn1iuhsG33XzFrC4QwkTdUnxOGdcPQOj-cuaEZeX-eA/edit#gid=0');
  query.send(drawDashboard);
}

function drawDashboard(response) {
  var data = response.getDataTable();

  //Asign units of 'mm' to data.
  var formatMS = new google.visualization.NumberFormat({
    pattern: '# mm'
  });

  // format into data mm..
  for (var colIndex = 1; colIndex < data.getNumberOfColumns(); colIndex++) {
    formatMS.format(data, colIndex);
  }

  var YearPicker = new google.visualization.ControlWrapper({
    controlType: 'NumberRangeFilter',
    containerId: 'filter_div',
    options: {
      filterColumnLabel: 'Year',
      ui: {
        cssClass: 'filter-date',
        format: {pattern: '0000'},
        labelStacking: 'vertical',
        allowTyping: false,
        allowMultiple: false
      }
    },
  });

  var MSLChart = new google.visualization.ChartWrapper({
    chartType: 'LineChart',
    containerId: 'chart_div',
    dataTable: data,
    options: {
      fontSize: '14',
      title: 'Timbucktoo Annual Mean Sea Level Summary',
      hAxis: {title: 'Year', format: '0000', maxValue: 2120},
      vAxis: {title: 'Height above Chart Datum (mm)', format:'###0'},
      height: 600,
      chartArea: {height: '81%', width: '85%', left: 100},
      legend: {position: 'in', alignment: 'end', textStyle: {fontSize: 13}},
      colors: ['blue'],
      trendlines: {
        0: {
          type: 'polynomial',
          degree: 2,
          color: 'green',
          visibleInLegend: true,
        },
        1: {
          type: 'linear',
          color: 'black',
          visibleInLegend: true,
        },
      },
      series: {
        0: { visibleInLegend: true },
        1: { visibleInLegend: false },
      },
    },
    view: {columns: [0,1,2]}
  });

  google.visualization.events.addOneTimeListener(MSLChart, 'ready', filterChange);

  function filterChange() {
    // get chart layout
    var chartLayout = MSLChart.getChart().getChartLayoutInterface();

    // get y-axis bounds
    var yAxisCoords = {min: null, max: null};
    var lineIndex = 0;
    var boundsLine = chartLayout.getBoundingBox('line#' + lineIndex);
    do {
      yAxisCoords.max = yAxisCoords.max || boundsLine.top;
      yAxisCoords.max = Math.min(yAxisCoords.max, boundsLine.top);
      yAxisCoords.min = yAxisCoords.min || (boundsLine.top + boundsLine.height);
      yAxisCoords.min = Math.max(yAxisCoords.min, (boundsLine.top + boundsLine.height));
      lineIndex++;
      boundsLine = chartLayout.getBoundingBox('line#' + lineIndex);
    } while (boundsLine !== null);

    // re-draw chart
    MSLChart.setOption('vAxis.viewWindow.max', chartLayout.getVAxisValue(yAxisCoords.max));
    MSLChart.setOption('vAxis.viewWindow.min', chartLayout.getVAxisValue(yAxisCoords.min));
    MSLChart.draw();
    google.visualization.events.addOneTimeListener(MSLChart.getChart(), 'ready', filterChange);
  }

  var dashboard = new google.visualization.Dashboard(
    document.getElementById('dashboard_div')
  ).bind(YearPicker, MSLChart).draw(data);
}

<script src="https://www.gstatic.com/charts/loader.js"></script>
<div id="dashboard_div">
  <div id="chart_div"></div>
  <div id="filter_div"></div>
</div>

---

note: it appears you're using an old load statement, to load google chart.
see above snippet for update...