Here is an excerpt from the output from the serial monitor, after driving around in circles in a car with the sensor on, using the unmodified adafruit sample code.
HMC5883 Magnetometer Test
------------------------------------
Sensor: HMC5883
Driver Ver: 1
Unique ID: 12345
Max Value: 800.00 uT
Min Value: -800.00 uT
Resolution: 0.20 uT
------------------------------------
X: -22.18 Y: 63.27 Z: -28.88 uT
Heading (degrees): 121.92
X: -22.09 Y: 63.27 Z: -28.57 uT
Heading (degrees): 121.85
X: -21.91 Y: 63.18 Z: -28.67 uT
Heading (degrees): 121.73
X: -21.73 Y: 63.45 Z: -28.67 uT
Heading (degrees): 121.51
X: -22.09 Y: 63.18 Z: -28.57 uT
Heading (degrees): 121.88
X: -22.64 Y: 63.36 Z: -28.57 uT
Heading (degrees): 122.26
X: -22.27 Y: 63.27 Z: -28.57 uT
Heading (degrees): 122.00
X: -22.55 Y: 63.36 Z: -28.47 uT
Heading (degrees): 122.19
X: -20.91 Y: 62.82 Z: -28.67 uT
Heading (degrees): 121.02
X: -19.27 Y: 62.36 Z: -32.45 uT
Heading (degrees): 119.78
X: -14.18 Y: 61.18 Z: -38.98 uT
Heading (degrees): 115.66
X: -15.36 Y: 60.73 Z: -40.20 uT
Heading (degrees): 116.80
X: -16.55 Y: 61.18 Z: -41.43 uT
Heading (degrees): 117.74
X: -17.82 Y: 60.82 Z: -41.22 uT
Heading (degrees): 118.93
X: -18.36 Y: 61.09 Z: -41.73 uT
Heading (degrees): 119.34
X: -18.55 Y: 61.00 Z: -41.33 uT
Heading (degrees): 119.52
X: -20.00 Y: 61.45 Z: -42.14 uT
Heading (degrees): 120.63
X: -18.73 Y: 61.55 Z: -42.45 uT
Heading (degrees): 119.53
X: -18.18 Y: 62.18 Z: -41.63 uT
Heading (degrees): 118.90
X: -18.27 Y: 62.64 Z: -42.55 uT
Heading (degrees): 118.87
X: -16.36 Y: 62.73 Z: -41.63 uT
Heading (degrees): 117.23
X: -14.27 Y: 62.27 Z: -40.82 uT
Heading (degrees): 115.51
X: -14.00 Y: 62.09 Z: -41.33 uT
Heading (degrees): 115.31
X: -10.91 Y: 62.36 Z: -42.14 uT
Heading (degrees): 112.53
X: -7.09 Y: 62.09 Z: -41.63 uT
Heading (degrees): 109.12
X: -8.55 Y: 60.73 Z: -45.31 uT
Heading (degrees): 110.62
X: -8.45 Y: 58.91 Z: -49.18 uT
Heading (degrees): 110.77
X: -4.36 Y: 56.09 Z: -48.78 uT
Heading (degrees): 107.05
X: -1.55 Y: 53.00 Z: -48.37 uT
Heading (degrees): 104.28
X: 0.36 Y: 48.55 Z: -49.29 uT
Heading (degrees): 102.18
X: 1.00 Y: 45.00 Z: -49.39 uT
Heading (degrees): 101.33
X: 1.27 Y: 40.36 Z: -48.88 uT
Heading (degrees): 100.80
X: -1.91 Y: 36.09 Z: -50.10 uT
Heading (degrees): 105.63
X: -2.82 Y: 32.64 Z: -49.49 uT
Heading (degrees): 107.54
X: -4.09 Y: 29.73 Z: -47.86 uT
Heading (degrees): 110.44
X: -7.64 Y: 26.00 Z: -47.96 uT
Heading (degrees): 118.97
X: -11.73 Y: 23.82 Z: -47.76 uT
Heading (degrees): 128.82
X: -15.00 Y: 22.91 Z: -46.53 uT
Heading (degrees): 135.82
X: -18.36 Y: 21.73 Z: -45.61 uT
Heading (degrees): 142.81
X: -21.36 Y: 22.00 Z: -43.78 uT
Heading (degrees): 146.76
X: -24.45 Y: 22.82 Z: -42.65 uT
Heading (degrees): 149.59
X: -27.00 Y: 24.82 Z: -37.86 uT
Heading (degrees): 150.02
X: -29.55 Y: 26.00 Z: -37.55 uT
Heading (degrees): 151.26
X: -30.82 Y: 27.91 Z: -36.43 uT
Heading (degrees): 150.44
X: -33.82 Y: 30.09 Z: -36.12 uT
Heading (degrees): 150.94
X: -35.73 Y: 32.55 Z: -35.10 uT
Heading (degrees): 150.27
X: -37.45 Y: 35.82 Z: -35.82 uT
Heading (degrees): 148.88
X: -36.18 Y: 38.73 Z: -33.57 uT
Heading (degrees): 145.66
X: -39.09 Y: 41.73 Z: -35.41 uT
Heading (degrees): 145.74
X: -38.82 Y: 44.36 Z: -34.69 uT
Heading (degrees): 143.79
X: -38.18 Y: 46.91 Z: -35.20 uT
Heading (degrees): 141.75
X: -37.82 Y: 49.00 Z: -35.71 uT
Heading (degrees): 140.27
X: -36.73 Y: 52.00 Z: -35.20 uT
Heading (degrees): 137.84
X: -36.18 Y: 53.91 Z: -34.18 uT
Heading (degrees): 136.47
X: -35.36 Y: 57.55 Z: -38.06 uT
Heading (degrees): 134.18
X: -35.64 Y: 59.55 Z: -37.14 uT
Heading (degrees): 133.50
X: -23.55 Y: 61.45 Z: -30.51 uT
Heading (degrees): 123.57
X: -23.45 Y: 62.64 Z: -28.88 uT
Heading (degrees): 123.13
X: -18.45 Y: 62.45 Z: -28.57 uT
Heading (degrees): 119.07
X: -16.09 Y: 63.27 Z: -28.37 uT
Heading (degrees): 116.87
X: -18.91 Y: 63.64 Z: -29.18 uT
Heading (degrees): 119.15
X: -21.45 Y: 64.00 Z: -25.92 uT
Heading (degrees): 121.14
X: -18.91 Y: 64.00 Z: -27.96 uT
Heading (degrees): 119.07
X: -14.64 Y: 62.64 Z: -26.43 uT
Heading (degrees): 115.76
X: -8.00 Y: 62.09 Z: -27.35 uT
Heading (degrees): 109.95
X: -7.64 Y: 63.00 Z: -29.39 uT
Heading (degrees): 109.52
X: -10.27 Y: 63.45 Z: -31.53 uT
Heading (degrees): 111.80
X: -11.09 Y: 62.91 Z: -36.63 uT
Heading (degrees): 112.60
X: -9.82 Y: 61.55 Z: -41.02 uT
Heading (degrees): 111.67
X: -6.36 Y: 63.00 Z: -39.39 uT
Heading (degrees): 108.37
X: -6.45 Y: 63.09 Z: -40.10 uT
Heading (degrees): 108.45
X: -5.91 Y: 63.09 Z: -40.41 uT
Heading (degrees): 107.96
X: -7.18 Y: 63.00 Z: -42.65 uT
Heading (degrees): 109.11
X: -7.45 Y: 63.27 Z: -42.35 uT
Heading (degrees): 109.32
X: -7.36 Y: 63.27 Z: -42.35 uT
Heading (degrees): 109.24
X: -7.27 Y: 63.00 Z: -42.14 uT
Heading (degrees): 109.19
X: -7.18 Y: 63.00 Z: -42.24 uT
Heading (degrees): 109.11
X: -7.09 Y: 63.09 Z: -42.24 uT
Heading (degrees): 109.02
X: -7.55 Y: 63.36 Z: -42.04 uT
Heading (degrees): 109.40
X: -7.36 Y: 62.82 Z: -42.24 uT
Heading (degrees): 109.29
X: -7.36 Y: 63.09 Z: -41.73 uT
Heading (degrees): 109.26
X: -7.55 Y: 62.82 Z: -42.45 uT
Heading (degrees): 109.45
X: -7.45 Y: 62.82 Z: -42.35 uT
Heading (degrees): 109.37
X: -7.55 Y: 63.18 Z: -42.76 uT
Heading (degrees): 109.42
X: -7.36 Y: 63.09 Z: -42.86 uT
Heading (degrees): 109.26
X: -7.73 Y: 63.09 Z: -42.55 uT
Heading (degrees): 109.59
X: -7.64 Y: 63.09 Z: -42.86 uT
Heading (degrees): 109.51
X: -7.55 Y: 63.09 Z: -42.96 uT
Heading (degrees): 109.43
X: -7.73 Y: 63.18 Z: -43.06 uT
Heading (degrees): 109.58
X: -7.73 Y: 62.91 Z: -42.76 uT
Heading (degrees): 109.61
X: -8.00 Y: 63.18 Z: -43.06 uT
Heading (degrees): 109.82
X: -7.91 Y: 62.64 Z: -43.37 uT
Heading (degrees): 109.80
X: -7.73 Y: 63.09 Z: -43.06 uT
Heading (degrees): 109.59
X: -8.00 Y: 63.09 Z: -43.16 uT
Heading (degrees): 109.83
X: -7.91 Y: 63.09 Z: -43.06 uT
Heading (degrees): 109.75
X: -8.09 Y: 63.00 Z: -43.27 uT
Heading (degrees): 109.92
X: -7.82 Y: 63.18 Z: -43.78 uT
Heading (degrees): 109.66
X: -8.09 Y: 63.00 Z: -43.57 uT
Heading (degrees): 109.92
X: -8.18 Y: 63.09 Z: -43.67 uT
Heading (degrees): 109.99
X: -8.09 Y: 63.18 Z: -43.78 uT
Heading (degrees): 109.90
X: -8.00 Y: 62.91 Z: -43.88 uT
Heading (degrees): 109.85
X: -8.18 Y: 62.91 Z: -43.47 uT
Heading (degrees): 110.02
X: -8.36 Y: 62.91 Z: -43.16 uT
Heading (degrees): 110.18
X: -8.64 Y: 63.18 Z: -43.67 uT
Heading (degrees): 110.39
X: -9.00 Y: 63.09 Z: -44.59 uT
Heading (degrees): 110.72
X: -9.00 Y: 63.18 Z: -45.10 uT
Heading (degrees): 110.71
X: -8.55 Y: 63.27 Z: -45.31 uT
Heading (degrees): 110.30
X: -8.64 Y: 63.55 Z: -45.00 uT
Heading (degrees): 110.34
X: -9.18 Y: 63.82 Z: -45.82 uT
Heading (degrees): 110.79
X: -8.91 Y: 63.27 Z: -45.00 uT
Heading (degrees): 110.62
X: -9.27 Y: 63.64 Z: -45.31 uT
Heading (degrees): 110.90
X: -8.91 Y: 63.55 Z: -45.20 uT
Heading (degrees): 110.59
X: -8.55 Y: 63.36 Z: -44.59 uT
Heading (degrees): 110.29
X: -8.64 Y: 63.64 Z: -44.69 uT
Heading (degrees): 110.33
X: -8.64 Y: 63.45 Z: -45.00 uT
Heading (degrees): 110.36
X: -8.91 Y: 63.45 Z: -44.90 uT
Heading (degrees): 110.60
X: -8.91 Y: 63.55 Z: -44.80 uT
Heading (degrees): 110.59
X: -8.73 Y: 63.18 Z: -45.20 uT
Heading (degrees): 110.47
X: -8.73 Y: 63.45 Z: -45.31 uT
Heading (degrees): 110.44
X: -8.45 Y: 63.45 Z: -45.20 uT
Heading (degrees): 110.19
X: -8.64 Y: 63.36 Z: -45.31 uT
Heading (degrees): 110.37
X: -8.91 Y: 63.45 Z: -45.92 uT
Heading (degrees): 110.60
X: -9.00 Y: 63.73 Z: -46.33 uT
Heading (degrees): 110.64
X: -9.18 Y: 63.64 Z: -46.12 uT
Heading (degrees): 110.82
X: -9.27 Y: 63.18 Z: -46.53 uT
Heading (degrees): 110.95
X: -9.27 Y: 63.73 Z: -45.92 uT
Heading (degrees): 110.88
X: -8.73 Y: 63.36 Z: -45.61 uT
Heading (degrees): 110.45
X: -8.73 Y: 63.18 Z: -46.43 uT
Heading (degrees): 110.47
X: -8.82 Y: 63.27 Z: -46.73 uT
Heading (degrees): 110.54
X: -9.18 Y: 63.18 Z: -47.45 uT
Heading (degrees): 110.87
X: -9.00 Y: 63.00 Z: -46.94 uT
Heading (degrees): 110.74
X: -8.73 Y: 63.18 Z: -47.45 uT
Heading (degrees): 110.47
X: -8.55 Y: 63.18 Z: -46.63 uT
Heading (degrees): 110.31
X: -8.27 Y: 63.45 Z: -47.35 uT
Heading (degrees): 110.03
X: -7.82 Y: 63.55 Z: -49.90 uT
Heading (degrees): 109.62
X: -6.73 Y: 63.27 Z: -50.61 uT