ui.html

<link rel="stylesheet" href="https://static.figma.com/api/figma-extension-api-0.0.1.css">
<h2>Bar Chart Sample</h2>
<p>
  Values:
  <input id="textbox" value="1, 3, 10, 28, 41, 87, 100, 114, 94, 68, 50, 23, 9, 3, 1">
</p>
<b id="errors"></b>
<p><button>Create Bar Chart</button></p>
<script>
  document.querySelector('button').onclick = () => {
    const text = document.querySelector('#textbox').value

    let numbers = text.split(',').map(x => +x)
    if (numbers.length < 2) {
      showError('Error: Must have at least two values');
      return
    }
    if (numbers.some(x => isNaN(x))) {
      showError('Error: All values must be numbers');
      return
    }

    parent.postMessage({ pluginMessage: numbers }, '*')
  }

  showError = errorMessage => {
    document.querySelector('#errors').textContent = errorMessage
  }
</script>