Allow negative values for both gauge and single-stat thresholds

ui/src/dashboards/components/CellEditorOverlay.js

@@ -155,7 +155,7 @@ class CellEditorOverlay extends Component {
     // If type === min, make sure it is less than the next threshold
     if (threshold.type === COLOR_TYPE_MIN) {
       const nextValue = Number(sortedColors[1].value)
-      allowedToUpdate = targetValueNumber < nextValue && targetValueNumber >= 0
+      allowedToUpdate = targetValueNumber < nextValue
     }
     // If type === max, make sure it is greater than the previous threshold
     if (threshold.type === COLOR_TYPE_MAX) {

ui/src/dashboards/components/Threshold.js

@@ -87,7 +87,6 @@ class Threshold extends Component {
           type="number"
           onChange={this.handleChangeWorkingValue}
           onBlur={this.handleBlur}
-          min={0}
         />
         <ColorDropdown
           colors={GAUGE_COLORS}