table.heatmap {
      border-collapse: collapse;
      width: 100%;
      text-align: center;
    }
    table.heatmap > th, td {
      border: 1px solid #ccc;
      padding: 4px;
      font-size: 12px;
    }
    table.heatmap > th {
      background: #f4f4f4;
    }
    .heat-0 { background-color: #ffffff; }  /* no activity */
    .heat-1 { background-color: #cce5ff; }
    .heat-2 { background-color: #99ccff; }
    .heat-3 { background-color: #66b3ff; }
    .heat-4 { background-color: #3399ff; }
    .heat-5 { background-color: #007bff; color: white; }
    .heat-6 { background-color: #0056b3; color: white; } /* max activity */