transforms filter breaks with one observation

Hi,

Theres an issue with the transform filter if there is only one observation. It just returns the chart axis. Below is a code to reproduce the issue.

library(data.table)
library(plotly)

# Transforms does not work if there is only one observation
x.data_exp <- data.table(Test = 'a', Exposure = 1)

plt <- plot_ly(height=400, transforms=list(list(type='filter', target=~Exposure, operation='>', value="-1"))) %>%
              add_bars(data = x.data_exp, x=~Test, y = ~Exposure) %>%
              layout(updatemenus = list(list(type='buttons', active=0, x=1.08, y=-0.1, xanchor='left', yanchor='bottom', direction='down', 
                                             buttons=list(
                                               list(label='all', method='restyle', args=list('transforms[0].value','-1')),
                                               list(label='1pc+', method='restyle', args=list('transforms[0].value','0.01')),
                                               list(label='3pc+', method='restyle', args=list('transforms[0].value','0.03'))
                                             ))))

I looked into the HTML and there is a part in the json data where "target":1. If you change this to "target":[1] the chart works as intended. So if there is only one observation maybe its outputting a scaler when it should be an array.

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8"/>
<style>body{background-color:white;}</style>
<script src="lib/htmlwidgets-1.5.4/htmlwidgets.js"></script>
<script src="lib/plotly-binding-4.10.0/plotly.js"></script>
<script src="lib/typedarray-0.1/typedarray.min.js"></script>
<script src="lib/jquery-1.11.3/jquery.min.js"></script>
<link href="lib/crosstalk-1.0.0/css/crosstalk.css" rel="stylesheet" />
<script src="lib/crosstalk-1.0.0/js/crosstalk.min.js"></script>
<link href="lib/plotly-htmlwidgets-css-2.5.1/plotly-htmlwidgets.css" rel="stylesheet" />
<script src="lib/plotly-main-2.5.1/plotly-latest.min.js"></script>

</head>
<body>
<div id="htmlwidget_container">
  <div id="htmlwidget-076f9323fea045959323" style="width:100%;height:400px;" class="plotly html-widget"></div>
</div>
<script type="application/json" data-for="htmlwidget-076f9323fea045959323">{"x":{"visdat":{"1628140951fe":["function () ","plotlyVisDat"],"16285be3752d":["function () ","data"]},"cur_data":"16285be3752d","attrs":{"16285be3752d":{"transforms":[{"type":"filter","target":{},"operation":">","value":"-1"}],"alpha_stroke":1,"sizes":[10,100],"spans":[1,20],"x":{},"y":{},"type":"bar","inherit":true}},"layout":{"height":400,"margin":{"b":40,"l":60,"t":25,"r":10},"updatemenus":[{"type":"buttons","active":0,"x":1.08,"y":-0.1,"xanchor":"left","yanchor":"bottom","direction":"down","buttons":[{"label":"all","method":"restyle","args":["transforms[0].value","-1"]},{"label":"1pc+","method":"restyle","args":["transforms[0].value","0.01"]},{"label":"3pc+","method":"restyle","args":["transforms[0].value","0.03"]}]}],"xaxis":{"domain":[0,1],"automargin":true,"title":"Test","type":"category","categoryorder":"array","categoryarray":["a"]},"yaxis":{"domain":[0,1],"automargin":true,"title":"Exposure"},"hovermode":"closest","showlegend":false},"source":"A","config":{"modeBarButtonsToAdd":["hoverclosest","hovercompare"],"showSendToCloud":false},"data":[{"transforms":[{"type":"filter","target":1,"operation":">","value":"-1"}],"x":["a"],"y":[1],"type":"bar","marker":{"color":"rgba(31,119,180,1)","line":{"color":"rgba(31,119,180,1)"}},"error_y":{"color":"rgba(31,119,180,1)"},"error_x":{"color":"rgba(31,119,180,1)"},"xaxis":"x","yaxis":"y","frame":null}],"highlight":{"on":"plotly_click","persistent":false,"dynamic":false,"selectize":false,"opacityDim":0.2,"selected":{"opacity":1},"debounce":0},"shinyEvents":["plotly_hover","plotly_click","plotly_selected","plotly_relayout","plotly_brushed","plotly_brushing","plotly_clickannotation","plotly_doubleclick","plotly_deselect","plotly_afterplot","plotly_sunburstclick"],"base_url":"https://plot.ly"},"evals":[],"jsHooks":[]}</script>
<script type="application/htmlwidget-sizing" data-for="htmlwidget-076f9323fea045959323">{"viewer":{"width":"100%","height":400,"padding":0,"fill":false},"browser":{"width":"100%","height":400,"padding":0,"fill":false}}</script>
</body>
</html>

Thanks,
Andrew

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