Slopegraph

.block

license: gpl-3.0
height: 940
border: no

README.md

Edward Tufte introduced Slopegraphs in his 1983 book The Visual Display of Quantitative Information. He describes them as "compar[ing] changes over time for a list of nouns located on an ordinal or interval scale".

This example recreates Tufte's original chart. Getting the labeling correct is a little tricky so it makes a perfect example of something that belongs in a layout.

d3.slopegraph.js

(function () {

  function _accessor ( s ) {
    return ( typeof s === 'function' ) ? s : function ( d ) { return d[ s ]; };
  }

  function crunch ( data, min_needed_dist, get_x, get_y, get_j ) {

    min_needed_dist = min_needed_dist || 0.5;

    // step 1: group the values
    var rollup_rows = {},
        rollup_cols = {},
        xs = [],
        js = [],
        values = [];

    for ( var di=0,dl=data.length; di<dl; di++ ) {
      var item = data[ di ],
          x = get_x( item ),
          j = get_j( item );

      item.y = get_y( item );

      rollup_rows[ j ] = rollup_rows[ j ] || {};
      rollup_rows[ j ][ x ] = item;

      rollup_cols[ x ] = rollup_cols[ x ] || [];
      rollup_cols[ x ].push( item );

      xs.push( x );
      js.push( j );
    }

    var xs = d3.set( xs ).values().sort( d3.ascending );
    var facts = d3.set( js ).values().map(function ( j ) {
      return xs.map(function ( x ) {
        return rollup_rows[ j ][ x ];
      });
    });

    // step 2: "fan out" label positions to remove overlaps
    for ( var col_id in rollup_cols ) {

      var column = rollup_cols[ col_id ].sort(function ( a, b ) {
        var r = get_y( a ) - get_y( b );
        if ( r === 0 ) {
          if ( get_j( a ) > get_j( b ) ) {
            return -1;
          }
          if ( get_j( a ) < get_j( b ) ) {
            return 1;
          }
        }
        return r;
      });

      var improoving = true,
          steps = 0;

      while ( improoving ) {

        var last_gap_size = null,
            smallest_gap = null,
            smallest_gap_size = -Infinity;

        // compute distances
        for ( var i=0,l=column.length; i<l; i++ ) {
          var item = column[ i ],
              prev = column[ i - 1 ],
              next = column[ i + 1 ];
          // space above
          if ( !prev ) {
            item._top = Infinity;
          }
          else {
            item._top = item.y - prev.y;
            // remember it if it was important
            if ( !smallest_gap || smallest_gap_size > item._top ) {
              smallest_gap = [ item, prev ];
              smallest_gap_size = item._top;
            }
          }
          // space below
          if ( !next ) {
            item._bottom = Infinity;
          }
          else {
            item._bottom = next.y - item.y;
            // remember it if it was important
            if ( !smallest_gap || smallest_gap_size > item._bottom ) {
              smallest_gap = [ next, item ];
              smallest_gap_size = item._bottom;
            }
          }
        }
        // find the smallest gap
        if ( smallest_gap_size >= min_needed_dist || /* no overlaps present */
             steps > 1000 /* we're going nowhere fast */ ) {
          break;
        }
        steps++;

        // push items apart, aiming toward empty spaces
        var t = smallest_gap[ 0 ]._bottom + smallest_gap[ 1 ]._top,
            a = isFinite( t ) ? smallest_gap[ 0 ]._bottom / t : 1,
            b = isFinite( t ) ? smallest_gap[ 1 ]._top    / t : 1,
            force = min_needed_dist / 4;

        smallest_gap[ 0 ].y += force * a;
        smallest_gap[ 1 ].y -= force * b;

        // stop doing this when labels stop overlapping
        improoving = ( smallest_gap_size >= ( last_gap_size || 0 ) );

        last_gap_size = smallest_gap_size;
      }

    }

    return facts;

  }


  d3.layout.slopegraph = function () {

    var get_x = _accessor( 'x' ),
        get_y = _accessor( 'y' ),
        get_j = _accessor( Number ),
        data = [],
        cached,
        min_needed_dist;

    var layout = function ( d ) {
      if ( arguments.length ) {
        cached = undefined;
        data = d;
        return layout;
      }
      return data;
    };

    layout.data = layout;

    layout.j = function ( d ) {
      if ( arguments.length ) {
        get_j = _accessor( d );
        return this;
      }
      return get_j;
    };

    layout.x = function ( d ) {
      if ( arguments.length ) {
        get_x = _accessor( d );
        return this;
      }
      return get_x;
    };

    layout.y = function ( d ) {
      if ( arguments.length ) {
        get_y = _accessor( d );
        return this;
      }
      return get_y;
    };

    layout.textHeight = function ( h ) {
      if ( arguments.length ) {
        min_needed_dist = h;
        cached = undefined;
        return this;
      }
      return min_needed_dist;
    };

    layout.left = function ( d ) {
      return [];
    };

    layout.right = function ( d ) {
      return [];
    };

    layout.pairs = function ( d ) {
      return cached || (cached = crunch( data, min_needed_dist, get_x, get_y, get_j ));
    };

    return layout;

  };

})();

data.csv

index.html

<!DOCTYPE html>
<meta charset='utf-8'>
<style>
svg {
  font: 13px serif;
}
.years {
  font-size: 15px;
}
.desc {
  font-size: 16px;
}
.lines path {
  fill: none;
  stroke: black;
  stroke-width: 1.1;
}
</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="d3.slopegraph.js"></script>
<script>
d3.csv( 'data.csv', parseRow, ready );

function parseRow ( d ) {
  return { 'year': +d.year,
           'country': d.country,
           'value': +d.value };
}

function ready ( data ) {

  var margin = { top: 70, right: 0, bottom: 40, left: 0 },
      width = 710 - margin.left - margin.right,
      height = 940 - margin.top - margin.bottom,

      y_dom  = d3.extent( data, d => d.value ).reverse()
      x_dom  = d3.extent( data, d => d.year )

      y = d3.scale.linear()
          .domain( y_dom )
          .range([ 0, height ]),
      x = d3.scale.linear()
          .domain( x_dom )
          .range([ 390, 570 ]),

      layout = d3.layout.slopegraph()( data )
          .j( 'country' ).y( 'value' ).x( 'year' )
          .textHeight( (y_dom[0] - y_dom[1]) / height * 14 ),

      textAlign = m => {
        return (d, i) => i ? 'start' : 'end';
      },
      textMargin = m => {
        return (d, i) => i ? m * 1 : m * -1;
      };

  var svg = d3.select( 'body' ).append( 'svg' )
      .attr( 'width', width + margin.left + margin.right )
      .attr( 'height', height + margin.top + margin.bottom )
    .append( 'g' )
      .attr( 'transform', `translate(${margin.left},${margin.top})` );

  svg.append( 'g' )
      .attr( 'class', 'years' )
      .selectAll( 'text' ).data( x_dom ).enter()
    .append( 'text' )
      .attr( 'x', x )
      .attr( 'dx', (d, i) => i ? 10 : -10 )
      .attr( 'y', -40 )
      .style( 'text-anchor', textAlign() )
      .text( String );

  var line = d3.svg.line()
      .x( d => x( d.year ) )
      .y( d => y( d.y ) );

  var pairs = svg.append( 'g' )
      .attr( 'class', 'lines' )
      .selectAll( 'g' )
      .data( layout.pairs() ).enter()
    .append( 'g' );

  pairs.append( 'path' )
      .attr( 'd', line );

  pairs.selectAll( '.country' )
      .data( d => d ).enter()
    .append( 'text' )
      .attr( 'class', 'country' )
      .attr( 'x', d => x( d.year ) )
      .attr( 'dx', textMargin( 48 ) )
      .attr( 'dy', '.32em' )
      .attr( 'y', d => y( d.y ) )
      .style( 'text-anchor', textAlign() )
      .text( d => d.country );

  pairs.selectAll( '.value' )
      .data( d => d ).enter()
    .append( 'text' )
      .attr( 'class', 'value' )
      .attr( 'x', d => x( d.year ) )
      .attr( 'dy', '.32em' )
      .attr( 'dx', textMargin( 10 ) )
      .attr( 'y', d => y( d.y ) )
      .style( 'text-anchor', textAlign() )
      .text( d => d.value.toFixed( 1 ) );

  svg.append( 'g' )
      .attr( 'class', 'desc' )
      .selectAll( 'text' )
      .data([ 'Current Receipts of Government as a'
            , 'Percentage of Gross Domestic'
            , 'Product, 1970 and 1979'
            ]).enter()
    .append( 'text' )
      .attr( 'y', (d,i) => i * 20 )
      .attr( 'dy', '-.32em' )
      .attr( 'x', 13 )
      .text( String );

}
</script>

thumbnail.png