title: Zingchart not showing crosshair plot-area guide-labels for series with only one point
body: <p>I have a multiple series (time) line chart. I define crosshair-x plot-area guide-labels on the individual series. If a series has more than one data element (point) the plot-area guide-labels for all points in that series display fine. If a series has only one point that specific series fails to show the plot-area guide labels. I am using the latest Chrome browser (63.0.3239.132)</p>
  
  <p>I have included the config below for the errant chart, but have also set up 2 examples on codepen. </p>
  
  <p><strong>Example with guide-labels not showing for the 1-point series</strong>
  <a href="https://codepen.io/iklotzko/pen/QQqEmp" rel="nofollow noreferrer">https://codepen.io/iklotzko/pen/QQqEmp</a></p>
  
  <p><strong>Example with guide-labels showing for both 2-point series</strong>
  <a href="https://codepen.io/iklotzko/pen/bLoeqp" rel="nofollow noreferrer">https://codepen.io/iklotzko/pen/bLoeqp</a></p>
  
  <p>The only difference between the above examples is S2 in the errant example has just 1 point.</p>
  
  <p><strong>The config for the errant example</strong>
  <strong>Series 1 (S1) has 2 points, Series 2 (S2) has 1 point. The guide labels for S2 fail to show in the plot area.</strong></p>
  
  <pre><code>var myConfig = 
      {
          "type":"line",
          "legend":{
              "layout":"1x5",
              "shadow":false,
              "adjustLayout":true,
              "margin-bottom":"17px",
              "margin-top":"0",
              "padding":"0",
              "align":"center",
              "background-color":"none",
              "vertical-align":"bottom",
              "border-top":"0 none",
              "border-right":"0 none",
              "border-bottom":"0 none",
              "border-left":"0 none"
          },
          "labels":{
              "visible":false
          },
          "marker":{
              "border-width":0,
              "border-color":"none",
              "shadow":false
          },
          "plot":{
              "line-width":1
          },
          "title":{
              "adjustLayout":true,
              "text":"S2-2pts, S2-pt",
              "font-size":12,
              "margin-top":2,
              "padding":0,
              "height":"10px",
              "bold":true,
              "border":"0 none",
              "font-color":"#333",
              "background-color":"white"
          },
          "subtitle":{
              "text":"subtitle",
              "color":"#444",
              "font-style":"italic",
              "font-size":12,
              "margin":0,
              "offset-y":-17,
              "padding":0,
              "height":"18px",
              "bold":false
          },
          "plotarea":{
              "margin":"dynamic 50 dynamic dynamic",
              "adjust-layout":true
          },
          "scaleX":{
              "items-overlap":true,
              "values":[1516942800000,1519362000000,1522382400000],
              "transform":{
                  "type":"date",
                  "all":"%d-%M-%Y",
                  "item":{
                      "visible":false
                  }
              },
              "item":{
                  "angle":-45,
                  "font-size":10
              }
          },
          "scaleY":{
              "guide":{
                  "lineStyle":"solid"
              },
              "format":"$%v",
              "thousands-separator":",",
              "label":{
                  "text":"Amount"
              }
          },
          "tooltip":{
              "visible":false
          },
          "crosshairX":{
              "lineColor":"#000",
              "plotLabel":{
                  "exact":false,
                  "multiple":true,
                  "font-color":"white",
                  "border-radius":"5px",
                  "text":"%t: $%v&lt;br /&gt;%data-custom (%k)",
                  "thousands-separator":",",
                  "decimals":2
              },
              "scaleLabel":{
                  "borderRadius":2,
                  "format":"$%v",
                  "thousands-separator":",",
                  "decimals":2
              },
              "marker":{
                  "size":5,
                  "type":"circle",
                  "backgroundColor":"transparent",
                  "border-color":"#888",
                  "border-width":"6px",
                  "border-alpha":0.4
              }
          },
          "series":[
              {
                  "values":[[1517893200000,19480], [1522382400000, 30000]],
                  "data-custom":["first-point", "second-point"],
                  "text":"S1",
                  "marker":{
                      "type":"star5",
                      "background-color":"blue",
                      "border-width":0,
                      "border-color":"none",
                      "size":6,
                      "shadow":false
                  },
                  "legend-marker":{
                      "type":"star5",
                      "backgroundColor":"blue",
                      "shadow":false,
                      "show-line":true,
                      "size":4,
                      "border-width":0,
                      "border-color":"none"
                  },
                  "guide-label":{
                      "background-color":"blue",
                      "border-color":"blue"
                  },
                  "lineColor":"blue"
              },
              {
                  "values":[],
                  "data-custom":[],
                  "text":"Empty Series",
                  "marker":{
                      "type":"diamond",
                      "background-color":"black",
                      "border-width":0,
                      "border-color":"none",
                      "size":6,
                      "shadow":false
                  },
                  "legend-marker":{
                      "type":"diamond",
                      "backgroundColor":"black",
                      "shadow":false,
                      "show-line":true,
                      "size":4,
                      "border-width":0,
                      "border-color":"none"
                  },
                  "guide-label":{
                      "background-color":"black",
                      "border-color":"black"
                  },
                  "lineColor":"black"
              },
              {
                  "values":[],
                  "data-custom":[],
                  "text":"Empty-Series",
                  "marker":{
                      "type":"square",
                      "background-color":"brown",
                      "border-width":0,
                      "border-color":"none",
                      "size":6,
                      "shadow":false
                  },
                  "legend-marker":{
                      "type":"square",
                      "backgroundColor":"brown",
                      "shadow":false,
                      "show-line":true,
                      "size":4,
                      "border-width":0,
                      "border-color":"none"
                  },
                  "guide-label":{
                      "background-color":"brown",
                      "border-color":"brown"
                  },
                  "lineColor":"brown"
              },
              {
                  "values":[],
                  "data-custom":[],
                  "text":"Empty-Series",
                  "marker":{
                      "type":"triangle",
                      "background-color":"green",
                      "border-width":0,
                      "border-color":"none",
                      "size":6,
                      "shadow":false
                  },
                  "legend-marker":{
                      "type":"triangle",
                      "backgroundColor":"green",
                      "shadow":false,
                      "show-line":true,
                      "size":4,
                      "border-width":0,
                      "border-color":"none"
                  },
                  "guide-label":{
                      "background-color":"green",
                      "border-color":"green"
                  },
                  "lineColor":"green"
              },
              {
                  "values":[[1517893200000,0]],
                  "data-custom":["first-point"],
                  "text":"S2",
                  "marker":{
                      "type":"circle",
                      "background-color":"red",
                      "border-width":0,
                      "border-color":"none",
                      "size":6,
                      "shadow":false
                  },
                  "legend-marker":{
                      "type":"circle",
                      "backgroundColor":"red",
                      "shadow":false,
                      "show-line":true,
                      "size":4,
                      "border-width":0,
                      "border-color":"none"
                  },
                  "guide-label":{
                      "background-color":"red",
                      "border-color":"red"
                  },
                  "lineColor":"red"
              }
          ],
          "id":"myChart"
      };
  </code></pre>